Full opinion text
BOOCHEVER, Circuit Judge: Jerry A. Whitworth, alleged participant in the infamous “Walker Family Spy Ring,” appeals his conviction for espionage and tax evasion. Whitworth was accused of selling classified Navy communications and cryptographic material to the Soviet Union through coconspirator John A. Walker, Jr., between 1975 and 1984. Whitworth was found guilty on twelve counts of a thirteen-count indictment in July 1986 following a four-month jury trial. United States District Court Judge John P. Vuka-sin, Jr., sentenced him to 365 years in prison, without the possibility of parole for sixty years, and fined him $410,000. Whitworth challenges his conviction on numerous grounds. He contends that (1) the government failed to comply with his discovery requests; (2) his espionage and tax evasion counts should not have been joined for trial; (3) his Miranda rights were violated; (4) he did not voluntarily consent to a search of his residence; (5) subsequent search warrants were over-broad and issued without probable cause; (6) certain letters signed “RUS” were not properly authenticated before being admitted into evidence; (7) erroneous evidentiary rulings were made during trial; (8) the evidence was insufficient to convict him for espionage; (9) certain tax fines were unauthorized; and (10) his prison sentence was excessive and in violation of his eighth amendment rights. Although we conclude that a statement by Walker was improperly admitted at trial, the government’s evidence against Whit-worth was overwhelming and the error was harmless. Whitworth’s conviction, his prison sentence, and three of his five tax fines therefore are affirmed. The fines imposed for filing false tax returns for 1979 and 1980 exceeded the statutory limit, however, and we reverse that portion of the judgment and remand to the district court for redetermination. Because our discussion of many of the issues is dependent on the facts, we shall relate them in some detail. I. Whitworth worked as a radioman on board the U.S.S. Arlington, a communications ship, and the U.S.S. Ranger, an aircraft carrier, between 1968 and 1970. Radiomen operate the communication centers aboard ships. The Navy uses cryptography in an attempt to conceal the content of the numerous directives, operation orders, and other messages sent and received at sea. Through this computerized process, the text of a communication is put in a form that in intelligible only to its intended recipients. Modern cryptographic systems consist of “logic,” a mathematical coding formula, and “keying material,” computer input in the form of cards, tapes, or printed material that alters the settings within the cryptographic logic. The keying material is changed each day and is subject to rigid custodial standards to provide maximum security. Whitworth, who was trained as a technical controller and held a top-secret security clearance, was privy to classified communications and cryptographic material. Whitworth attended radioman instructor school in San Diego in 1970 following his tour of duty aboard the U.S.S. Ranger. There, Whitworth first met Walker, who at the time was the school’s chief instructor. The men soon became best friends. In February 1973, Whitworth was granted a transfer to Diego Garcia Island, a new communications center in the Indian Ocean. He served as a radioman at Diego Garcia until opting to be discharged from the Navy in June 1974. Whitworth then returned to San Diego and worked part-time as a commercial pilot. By 1974, Walker had been selling cryptographic material and other Navy secrets to the Soviet Union for six years. He “began to contemplate [Whitworth] as a possible recruit to help me in the spying business.” Walker testified that he finally made his “sales pitch” to Whitworth in September 1974 in a San Diego restaurant: We went to a secluded portion of the bar ... and I told him I was interested in using him on an illegal activity, and that even discussing it was illegal.... He was excited and was interested in hearing what I had to say.... So I told him I’d been involved in selling classified material for a number of years, and it was profitable, and I could build him into the — into the sale_ He responded affirmatively, that he would be interested.... [H]e was mostly curious as to who the buyers were. I told him it was — that I wasn’t sure. I was working with people I had met, possibly could have been organized crime, Mafia, but that the buyers were allied countries, such as Israel, or private defense organizations, such as Jane’s Fighting Ships. Whitworth, who was a civilian at the time, agreed to rejoin the Navy and steal messages and cryptographic material for delivery to Walker. The classified information was to be passed on to the buyers by Walker and Whitworth was to be paid one-half of the cash proceeds. Whitworth was expected to receive $2,000 to $4,000 per month for spying. Despite numerous inquiries by Whitworth about the identity of the buyers, Walker said that he never told him that they were dealing with the Soviet Union. Whitworth reenlisted in the Naval Reserve in October 1974 and rejoined the regular Navy one month later in order to attend an advanced satellite communications training school in New Jersey. In February 1975, shortly before graduating from satellite school and commencing a second tour of duty at the Diego Garcia communications station, Whitworth met with Walker in Norfolk, Virginia. Walker paid Whitworth a $4,000 “inducement” and instructed him “to try to photograph or assemble cryptographic material and anything else sensitive that came across his desk.” Whitworth used his position as supervisor of the Diego Garcia technical control facility to assemble and photograph classified messages and cryptographic material. In April 1976, Whitworth again met Walker in Norfolk and delivered the material he had stolen during his one-year assignment at the communications center. Walker paid Whitworth $18,000 for the delivery from cash Walker had received from his Soviet contact. Between June 1976 and July 1978, Whit-worth worked aboard the aircraft carrier U.S.S. Constellation and supervised the use of cryptographic equipment and material. Whitworth continued to copy classified information and, while on leave, personally delivered it to Walker in August 1976, January, April, August, and November 1977, and February and July 1978. Whitworth received a total of $76,000 while working on the Constellation. Walker, whose spy role was limited to that of go-between after retiring from the Navy in 1976, relayed the material to the Soviets semiannually through a series of package “drops” and face-to-face meetings. He received cash and instructions in exchange. Whitworth next served on the U.S.S. Niagara Falls, a supply ship, in August 1978. During his one-year tour as chief radioman, Whitworth was the custodian of a large amount of cryptographic material used on a number of ships. He photographed classified information and delivered it to Walker in September and December 1978 and May and August 1979 in exchange for $24,-000. Whitworth requested shore duty and was assigned to the Alameda, California Naval Air Station in September 1979. He was in charge of the telecommunications center at Alameda for most of the next three years. Whitworth made deliveries to Walker in January, June, and November 1980, July 1981, and January and September 1982. Sporadic payments totaling $210,000 were made by the Soviets through Walker. This included a $100,000 payment in June 1980 covering deliveries made from the Niagara Falls and an additional $10,000 in July 1981 for a van Whitworth used to photograph cryptographic material in the parking lot of the Alameda Air Station. Following shore duty, Whitworth worked on board the aircraft carrier U.S.S. Enterprise between October 1982 and his retirement in October 1983. He delivered classified messages and cryptographic material from the Enterprise in June 1983, but no money was exchanged. Whitworth met Walker in July 1983 and January 1984, but again was not paid. At a meeting in Vienna in February 1984, Walker told his Soviet contact that Whitworth, though retired from the Navy, was looking for a civil service job with classified information access so that he could continue spying. The contact informed Walker that the messages delivered the previous June were photographed out-of-focus and unusable. Whitworth, who had retained copies of the classified messages he stole from the Enterprise, rephotographed the illegible ones. He gave them to Walker in Norfolk in April 1984, shortly before a scheduled drop with the Soviets near Washington, D.C. Walker included these messages in the drop, along with other classified Navy documents supplied by his son, Michael, whom Walker had recruited into the spy ring in 1983. Although Walker picked up money in the exchange, he was instructed that Whitworth was not to be paid. Whit-worth was shocked by this news, and reminded Walker that he had not yet been paid for an entire year of messages from the Enterprise. Three weeks later, the San Francisco FBI office received a typewritten letter dated May 7, 1984, which stated: Dear Sir: I have been involved in espionage for several years, specifically I’ve passed along Top Secret Cryptographic Keylists for military communications, Tech Manuals for same, Intelligence Messages, and etc. I didn’t know that the info was being passed to the USSR until after I had been involved a few years and since then I’ve been remoreseful [sic] and wished to be free. Finally I’ve decided to stop supplying material — my contact doesn’t know of my decision. Originally I was told I couldn’t get out without approval, this was accompanied with threats. Since then I believe the threats were a bluff. At any rate the reason for this letter is to give you (FBI) an opportunity to break what brobably [sic] is a significant espionage system. (I know that my contact has recurited [sic] at least three other members that are actively supplying highly classified material). (I have the confidence of my contact). I pass the material to my contact (a US citizen) who in turn passes the material to a contact overseas (his actual status— KGB or whatever — I don’t know). That is not always the case tho, sometimes US locations are used. A US location is always used to receive instructions and money. If you are interested in this matter you can signal me with an Ad in the Los Angeles Times Classified Section.... What I would expect to cooperate is complete immunity from prosecution and absolutely no public disclosure of me or my identity. I will look for an Ad in Monday editions only for the next four weeks.... Sincerely, RUS The FBI responded by placing an ad in the Times on May 21 requesting “RUS” to call or write and stating that it was “considering your offer.” A second “RUS letter,” dated May 21, 1984, stated: I saw your note today and was encouraged, however, I’m not going to call for obvious reasons. I’ll admit that my most earnest desire is to talk to someone (like yourself) about my situation, but I feel that I’m unable to trust any kind of personal contact — phone included_ It would certainly be nice for people in my predicament to have a means of confidential consultation with someone in a position of authority without the possibility of arrest. My contact will be expecting more material from me in a few months, if I don’t I’m not sure what his response will be. I’m going to come clean with him at that time (assuming no deal is made with you) and tell him I’m finished with the “business”. And then get on with my life. More info on him: he has been in “the business” for more than 20 years and plans to continue indefinitely.... The FBI offered to meet with RUS in ads placed on June 4 and 11. In a third letter, postmarked June 18, RUS declined the meeting, and stated that he would write again “in a week or two.” The FBI did not hear from RUS and placed an ad on August 13, 1984, suggesting a meeting at “a neutral site,” Ensena-da, Mexico. RUS responded that day with a fourth and final letter: I saw your note in todays [sic] LA Times. Since my last note to you I’ve done a lot of serious thinking and have pretty much come to the conclusion that it would be best to give up on the idea of aiding in the termination of the espionage ring previously discussed. To think I could help you and not make my own involvement known to the public, I believe is naive_ I have great difficulty in coming forth, particularly, since the changes of my past involvement ever being known is extremenely [sic] remote, as long as I remain silent.... The above notwithstanding, I’ll think about a meeting in Ensenada. Funds are not the problem. My contact is pressing for more material, but so far no real problems have occurred. I haven’t explicitly told him, I’m no longer in the business. The government contended at trial that Whitworth authored the RUS letters. After unsuccessfully challenging their admissibility into evidence, Whitworth’s counsel conceded the issue in closing argument, stating bluntly: “I’m not going to quibble with you about the postmarks or the typewriter ball or the stamps or the envelopes or the paper. The writer of the RUS letters was Jerry Whitworth.” On August 14, one day after sending the final RUS letter to the FBI, Whitworth wrote to Walker and announced his “resignation” from espionage. This decision was relayed at a face-to-face meeting with the Soviets in January 1985, who assured Walker that Whitworth would still be paid for the year’s worth of messages from the Enterprise. Walker told Whitworth in a February letter that payment was forthcoming. He expected a large amount of cash in the Soviets’ next drop, scheduled for May 1985 near Washington, D.C. That drop was never completed. Walker’s ex-wife, Barbara, who had been aware of his spying activities since shortly after they began in 1968, finally reported Walker to the FBI in November 1984. Walker’s daughter, Laura, provided corroborating information, and an investigation commenced in early 1985. A telephone wiretap revealed that Walker had a trip planned for May 19, 1985, and the FBI conducted fulls-cale surveillance of him that day. Walker was observed driving in rural Montgomery County, Maryland for a number of hours. He was arrested at a motel early the next morning, May 20, after a package he left near a utility pole was found to contain classified Navy documents stolen by his son, Michael, two letters from Whitworth, and a letter from Walker to a Soviet agent referring to Whitworth by a code letter. Soviet diplomat Aleksey Tka-chenko also was seen near the drop point on May 19, but apparently alerted to the surveillance, he neither attempted to pick up Walker’s package nor delivered one of his own. A search of Walker’s home pursuant to a warrant on May 20 uncovered additional evidence, including cryptographic material, classified documents and communications, travel records, and a miniature camera. Whitworth’s handwriting and fingerprints were found on numerous items. That same day, FBI agents Peterson and McEl-wee spoke with Whitworth at his mobile home in Davis, California. Peterson informed Whitworth of Walker’s arrest for espionage, asked if he knew about it, and accused him of being involved. Stunned, Whitworth replied, “you’re talking about some serious stuff.” Whitworth was given Miranda warnings and signed a written waiver. He talked about his relationship with Walker in a narrative manner for approximately ninety minutes. During this discussion, Whit-worth’s wife, Brenda, was heard arriving home. When Whitworth requested to see her, the agents stated that they preferred to complete the interview without interruption and that other agents were waiting to talk with Brenda separately. FBI Agents Griego and Ambrosavich questioned Brenda about Walker outside the mobile home before taking her to a local restaurant. Agents Peterson and McEIwee continued to interview Whitworth. Peterson produced one of the RUS letters and accused Whitworth of writing it. When asked whether he was familiar with the letter, Whitworth said he did not want to answer the question. Peterson requested consent to search the residence; Whitworth replied that he would like to “speak with someone first.” Peterson next asked if he would take a polygraph examination. Whitworth responded by invoking his right to counsel, and the agents halted their questioning. Peterson then called the FBI’s Sacramento office for instructions on how to proceed. Whitworth, who was sitting with McEIwee in an adjacent room, asked him what was likely to happen. McEIwee replied that the agents’ superiors might authorize them to arrest Whitworth or seek a search warrant. McEIwee said he believed that sufficient cause existed to issue a warrant and that if one was sought, the mobile home would have to be secured in the interim to prevent the destruction of evidence. At that point, Whitworth said that he would allow the agents to search the mobile home. McEIwee explained that because of his earlier refusal, accepting Whit-worth’s “consent” could be problematic and would require a supervisor’s approval. An FBI official in Sacramento was contacted by telephone, and twenty minutes later the agents were given permission to accept Whitworth’s consent. Whitworth then signed a written consent form and a search was conducted. Among the items seized was a Monday edition of the Los Angeles Times and an unfinished letter that Whit-worth was writing to Walker when the agents arrived. The mobile home was searched again eleven days later, on May 31, 1985. This search, conducted pursuant to a warrant, uncovered two miniature cameras, seventy-six unused rolls of film, another Monday edition of the Times, and a copy of a telegram from Whitworth to Walker. On June 5, a commercial storage unit rented by Whitworth also was searched pursuant to a warrant. Whitworth was arrested on June 3, 1985. He was the fourth and final member of the “Walker Family Spy Ring” to be taken into custody, following John Walker, his son Michael, and his brother Arthur, who worked for a defense contractor. In December 1985, a third superseding indictment charged Whitworth with one count of conspiracy to deliver national defense information to a foreign government, 18 U.S.C. § 794(e); six counts of delivery of national defense information to a foreign government, 18 U.S.C. § 794(a); one count of unlawfully obtaining national defense information, 18 U.S.C. § 793(b); four counts of making false tax returns, 26 U.S.C. § 7206(1); and one count of conspiracy to defraud the United States, 18 U.S.C. § 371. Whitworth’s espionage and tax counts were consolidated. The district court considered a number of pretrial motions, and trial began on March 24, 1986 amid extensive national publicity. The government’s key witness was Walker, who testified in accordance with an October 1985 plea agreement. Whitworth’s defense was based on a lack of knowledge that the information passed to Walker was being relayed to the Soviet Union. On July 24, after deliberating for ten days, the jury found Whitworth guilty on each count of the indictment except that of unlawfully obtaining national defense information, on which no verdict was returned. On August 28, the court sentenced Whit-worth to 180 years in prison on each of the seven espionage counts, three years on each of the four tax evasion counts, and five years on the conspiracy to defraud the government count. Fines of $100,000 on each of the tax evasion counts and $10,000 on the conspiracy to defraud count also were imposed. The court ordered some of the prison terms to be served concurrently and others consecutively. Summarizing, Judge Vukasin announced that he was sentencing Whitworth, who was forty-seven years old at the time, to “a cumulative term of 365 years in custody, a total fine of $410,000 ... [and] a minimum of 60 years in custody before he will be considered eligible for parole. The court did not impose this sentence lightly.” Whitworth filed a timely notice of appeal. II. On appeal, Whitworth alleges numerous errors by the district court. Few of these claims call for extended analysis, and none requires reversal of his espionage and tax evasion convictions. A. Discovery Before and during trial, Whitworth filed discovery requests pursuant to Fed.R.Crim. P. 16, the Jencks Act, 18 U.S.C. § 3500 (1982), and the Supreme Court’s decision in Brady v. Maryland, 373 U.S. 83, 83 S.Ct. 1194, 10 L.Ed.2d 215 (1963). Whitworth’s initial motion, filed in September 1985, requested discovery in eighteen broad areas. His second motion, in January 1986, contained fifteen requests. The court granted the motions to a large extent, and the government made extensive disclosures. Whitworth contends that he was denied due process and a fair trial by the government’s failure to comply fully with these requests and by the procedure used by the court to determine that certain material was not discoverable. Rule 16(a)(1)(A) allows a defendant to inspect and copy “the substance of any oral statement which the government intends to offer in evidence at the trial made by the defendant whether before or after arrest in response to interrogation by any person then known to the defendant to be a government agent.” Similarly, the Jencks Act permits discovery of statements by government witnesses relating to their trial testimony. 18 U.S.C. § 3500(b). Under Brady, the prosecution is required to disclose “evidence favorable to an accused upon request ... where the evidence is material either to guilt or to punishment.” 373 U.S. at 87, 83 S.Ct. at 1196-97. This includes evidence relating solely to the credibility of a key government witness. Giglio v. United States, 405 U.S. 150, 154-55, 92 S.Ct. 763, 766, 31 L.Ed.2d 104 (1972). We review for an abuse of discretion the application of Rule 16, United States v. Givens, 767 F.2d 574, 583 (9th Cir.), cert. denied, 474 U.S. 953, 106 S.Ct. 321, 88 L.Ed.2d 304 (1985), and the Jencks Act, United States v. Augenblick, 393 U.S. 348, 355, 89 S.Ct. 528, 533, 21 L.Ed.2d 537 (1969). Alleged Brady violations are reviewed de novo and require reversal only where there is a reasonable possibility that the evidence withheld could have materially affected the verdict. United States v. Lehman, 792 F.2d 899, 901 (9th Cir.), cert. denied, 479 U.S. 868, 107 S.Ct. 232, 93 L.Ed.2d 158 (1986). Before considering Whitworth’s third discovery request, filed in May 1986, Judge Vukasin announced that “all discovery orders heretofore issued by this court in connection with this case have been complied with.” With the exception of one meritless objection, Whitworth did not challenge the district court’s decisions on his first two motions. Some of these earlier discovery rulings were based on in camera review of the material. Whitworth did not object to this procedure, and in fact requested it in his third motion, which sought additional disclosures. On appeal, however, Whit-worth contends that his rights were violated by this process because it left him “in the blind” with no way of knowing whether the court’s rulings on his final request were correct. Although we realize that the in camera screening prevented Whitworth from knowing exactly which statements the court allowed the government to withhold, it was nevertheless incumbent upon him to make at least some kind of record for our review. Whitworth did not make an objection or request a ruling from Judge Vukasin regarding the procedure. He likewise has failed to even suggest the source or subject of any statement to which he was entitled. Given this record and the discovery that was allowed, we find no error. B. Joinder and Severance Whitworth claims that the eight espionage counts and five tax counts charged in his indictment were misjoined for trial. Under Fed.R.Crim.P. 8(a), offenses may be joined if they are either (1) of the same or similar character; (2) based on the same act or transaction; or (3) based on two or more acts or transactions connected together or constituting parts of a common scheme or plan. Misjoinder of charges is a question of law reviewed de novo. United States v. Smith, 795 F.2d 841, 850 (9th Cir.1986), cert. denied, — U.S. —, 107 S.Ct. 1964, 95 L.Ed.2d 535 (1987). In a pretrial motion, Whitworth argued that the tax offenses were not congruent with the espionage conspiracy because they were distinct in time and based on evidence from different witnesses. Yet he acknowledged that the money allegedly received in exchange for classified information was the same as that involved in the tax charges. The government contended, and the district court agreed, that the offenses were performed as part of a common plan and that proof of the tax counts was inextricably intertwined with the proof of the espionage charges. Judge Vukasin correctly noted that where tax evasion flows directly from other criminal activity and such evasion results in large part from the necessity of concealing the illegal proceeds of that activity, joinder of the substantive and tax charges is proper under Rule 8(a). See United States v. Bennett, 702 F.2d 833, 835 (9th Cir.1983); United States v. Kenny, 645 F.2d 1323, 1344 (9th Cir.), cert. denied, 452 U.S. 920, 101 S.Ct. 3059, 69 L.Ed.2d 425 (1981). Whitworth’s reliance on United States v. Halper, 590 F.2d 422 (2d Cir.1978), is misplaced because it was conceded in that case that “the sums charged in the income tax evasion indictment were not the same funds embraced in the Medicaid fraud indictment.” Id. at 429. Having found that initial joinder was proper, we address the related issue of whether the court abused its discretion by denying Whitworth’s pretrial motion for severance. Fed.R.Crim.P. 14 provides in part: If it appears that a defendant or the government is prejudiced by a joinder of offenses or of defendants in an indictment or information or by such joinder for trial together, the court may order an election or separate trials of counts, grant a severance of defendants or provide whatever other relief justice requires. In United States v. Armstrong, 621 F.2d 951, 954 (9th Cir.1980), we stated that “join-der is the rule rather than the exception.” A denial of severance will be upheld absent a showing that “joinder was so manifestly prejudicial that it outweighed the dominant concern with judicial economy and compelled exercise of the court’s discretion to sever.” Id.; accord Smith, 795 F.2d at 850; Kenny, 645 F.2d at 1345. Whitworth contends that he desired to testify concerning the espionage counts but wanted to assert his privilege against self-incrimination with respect to the tax evasion charges. To justify severance on this ground, a defendant must demonstrate that he has important testimony to give concerning some counts and a strong need to refrain from testifying on others. Armstrong, 621 F.2d at 954; United States v. Nolan, 700 F.2d 479, 483 (9th Cir.), cert. denied, 462 U.S. 1123, 103 S.Ct. 3095, 77 L.Ed.2d 1354 (1983). We agree that Whitworth failed to meet this burden. His defense to the espionage charges was based on a lack of knowledge that the Soviet Union was the buyer of the stolen messages and cryptographic material. It would not have been inconsistent with this strategy for Whitworth to be cross-examined regarding receipt of the unreported cash. C. Miranda Violations Whitworth also submits that statements made to FBI agents at his mobile home on May 20, 1985, the day of Walker’s arrest, were taken in violation of Miranda v. Arizona, 384 U.S. 436, 86 S.Ct. 1602, 16 L.Ed.2d 694 (1966), and thus were inadmissible at trial. The district court denied Whitworth’s suppression motion following a three-day evidentiary hearing in December 1985. Whitworth and FBI agents Peterson, McElwee, and Griego testified at the hearing, and a declaration by Whit-worth’s wife, Brenda, was received. The court based its ruling on two grounds. First, it found that Miranda warnings simply were unnecessary in this case because Whitworth was not “in custody” at the time he spoke to the agents. Oregon v. Mathiason, 429 U.S. 492, 495, 97 S.Ct. 711, 713-14, 50 L.Ed.2d 714 (1977) (per curiam). Second, assuming that Whit-worth was subjected to custodial interrogation, the court concluded that he was adequately apprised of his rights and made the statements voluntarily. Because the record discloses a valid waiver by Whit-worth, we need not address the “custody” question. United States v. Barrett, 703 F.2d 1076, 1087 (9th Cir.1983). The issue whether a defendant knowingly and voluntarily waives his Miranda rights involves a factual inquiry into the totality of the circumstances and the defendant’s state of mind. United States v. Doe, 819 F.2d 206, 209 (9th Cir.1985). We review the trial court’s finding for clear error. Id. McElwee testified that he and Peterson arrived at Whitworth’s residence at approximately 11:30 a.m., and that they sat down at a table with Whitworth at 11:38. Whit-worth agreed to talk to the agents after Peterson informed him of Walker’s arrest and accused him of being involved in espionage. Before any questions were answered regarding his relationship with Walker, however, Whitworth was informed of his Miranda rights both orally and in writing. An interview report filed by Peterson indicated that Whitworth signed a waiver form at 11:50 a.m., twelve minutes after sitting down with the agents, and did not ask to speak with a lawyer until 1:38 p.m. McEl-wee added that in accordance with Edwards v. Arizona, 451 U.S. 477, 484-85, 101 S.Ct. 1880, 1884-85, 68 L.Ed.2d 378 (1981), all questioning ceased when Whit-worth requested counsel, and resumed only when Whitworth asked what the FBI office was likely to advise the agents to do. Whitworth acknowledged reading and signing the Miranda waiver at 11:50 a.m., but testified that the agents had been in his house for thirty to sixty minutes by then. Whitworth said he believed he had no option at that point other than to sign the form because the agents “were putting a great deal of pressure on me continuously and psychologically I was beaten down and when I signed I signed for relief not from free will.” He also recalled asking for a lawyer as soon as Peterson accused him of being involved in espionage with Walker. Judge Vukasin, presented with conflicting facts, credited the testimony of Peterson and McElwee over that of Whit-worth in making his findings. He found that Miranda warnings were given “almost immediately” after Peterson’s initial accusatory remark, and that Whitworth’s later statements “were apparently made voluntarily.” Although he did not specifically address the request for counsel, “[i]mplicit in the district judge’s denial of the suppression motion was a finding that he believed the agent’s direct testimony that [the defendant] initiated the conversation.” United States v. Most, 789 F.2d 1411, 1416 (9th Cir.1986). The court’s finding that Whitworth knowingly and voluntarily waived his Miranda rights is not clearly erroneous and we hold that his statements were properly admitted. D. Consent Search A warrantless search of Whitworth’s residence was conducted on May 20, 1985, the day he was interviewed by the FBI. Whit-worth claims that he was subjected to “psychological warfare” by the agents and that his consent to the search was involuntary. He challenges the district court’s denial of his pretrial motion to suppress the items seized. In United States v. Al-Azzawy, 784 F.2d 890 (9th Cir.1985), cert. denied, 476 U.S. 1144, 106 S.Ct. 2255, 90 L.Ed.2d 700 (1986), we discussed the standards to be applied in reviewing consent searches: The government has the burden of demonstrating that consent to a warrant-less search was voluntary. Voluntariness is a question of fact to be determined from all the surrounding circumstances. A trial court’s finding on volun-tariness should not be overturned unless it is clearly erroneous. Id. at 895 (citations omitted). Consent to search is “voluntary” if it is “the product of an essentially free and unconstrained choice by its maker.” Schneckloth v. Bustamonte, 412 U.S. 218, 225, 93 S.Ct. 2041, 2047, 36 L.Ed.2d 854 (1973) (quoting Culombe v. Connecticut, 367 U.S. 568, 602, 81 S.Ct. 1860, 1879, 6 L.Ed.2d 1037 (1961)). When viewing the surrounding circumstances, “there is no single controlling criterion.” United States v. Agosto, 502 F.2d 612, 614 (9th Cir.1974) (per curiam). Judge Vukasin found “scant support” for Whitworth’s argument that the agents employed patently coercive measures in order to obtain his consent to the search. Although the court acknowledged that Whitworth experienced a degree of discomfort while talking to the agents and being separated from his wife, it determined that no threats were made, no weapons were drawn, Miranda warnings were given, and Whitworth was told that he was not under arrest. He also read and signed a waiver form stating that consent to search was not required, and testified that he was aware of this right. McElwee’s statement indicating that a search warrant would likely be sought and the mobile home secured could not have, by itself, rendered Whitworth’s consent involuntary as a matter of law. Agosto, 502 F.2d at 614; see United States v. Salvador, 740 F.2d 752, 757 (9th Cir.1984), cert. denied, 469 U.S. 1196, 105 S.Ct. 978, 83 L.Ed.2d 980 (1985). Judge Vukasin rejected the notion that Whitworth or his wife were threatened with unreasonable detention if he refused to consent. Cf. United States v. Ocheltree, 622 F.2d 992, 994 (9th Cir.1980). In view of the surrounding circumstances, we affirm the district court’s finding that the authorization to search was voluntary. E. Search Warrants Searches of Whitworth’s residence and a commercial storage unit were conducted by the FBI pursuant to warrants on May 31 and June 5, respectively. Whitworth claims that these warrants were issued without probable cause and were over-broad. The government maintains that the warrants were valid, but admits that reference to the earlier consent search of the mobile home was intentionally omitted from the supporting affidavits submitted to the magistrate. In Illinois v. Gates, 462 U.S. 213, 103 S.Ct. 2317, 76 L.Ed.2d 527 (1983), the Supreme Court stated that a magistrate’s finding of probable cause is to be given great deference: The task of the issuing magistrate is simply to make a practical, commonsense decision whether, given all the circumstances set forth in the affidavit before him, including the “veracity” and “basis of knowledge” of persons supplying hearsay information, there is a fair probability that contraband or evidence of a crime will be found in a particular place. And the duty of a reviewing court is simply to ensure that the magistrate had a “substantial basis for ... con-cludfing]” that probable cause existed. Id. at 238-39, 103 S.Ct. at 2332 (quoting Jones v. United States, 362 U.S. 257, 271, 80 S.Ct. 725, 736, 4 L.Ed.2d 697 (1960)); United States v. Calabrese, 825 F.2d 1342, 1348-49 (9th Cir.1987). This court reviews de novo whether a warrant describes the items to be seized with sufficient particularity. United States v. Fannin, 817 F.2d 1379, 1381 (9th Cir.1987). Based on all of the information presented to the magistrate, we are convinced that there was probable cause to issue the search warrants and that they were sufficiently narrow in scope. The affidavit supporting each warrant quoted statements by two informants that Walker admitted being involved in an espionage ring that included a Navy associate named Gerry or Jerry “Wentworth.” Both informants claimed to have personal knowledge of Walker’s activities and to have been trusted with confidences by him. Their testimony was corroborated by Arthur Walker, who told FBI agents that Whit-worth was involved in espionage. The affidavits also referred to the contents of the intercepted drop package, the search of Walker’s house, the “RUS letters,” and statements by Whitworth and his wife. A warrant need not describe the objects of a search in elaborate detail. United States v. Hayes, 794 F.2d 1348, 1354 (9th Cir.1986), cert. denied, 479 U.S. 1086, 107 S.Ct. 1289, 94 L.Ed.2d 146 (1987). The magistrate authorized the agents in this case to search for fifteen types of evidence relating to the crimes of espionage and fraud against the United States. The supporting affidavit was attached to and incorporated by reference in both warrants. Given the reasonable specificity of the warrants and affidavits regarding the items described and the offenses alleged, the presence of a “catch-all” provision at the end of the warrants allowing seizure of “other information or objects showing violations of the above statutes” did not render them overbroad. Fannin, 817 F.2d at 1383-84; see United States v. Spilotro, 800 F.2d 959, 964, 967 (9th Cir.1986). A finding of facial validity of the search warrants does not end our inquiry. Whit-worth further contends that he was improperly denied an evidentiary hearing pursuant to Franks v. Delaware, 438 U.S. 154, 98 S.Ct. 2674, 57 L.Ed.2d 667 (1978), to challenge the veracity of certain factual statements in the affidavits supporting the warrants. In Franks, the Court held that: [Wjhere the defendant makes a substantial preliminary showing that a false statement knowingly and intentionally, or with reckless disregard for the truth, was included by the affiant in the warrant affidavit, and if the allegedly false statement is necessary to the finding of probable cause, the Fourth Amendment requires that a hearing be held at the defendant’s request. 438 U.S. at 155-56, 98 S.Ct. at 2676. Similarly, a defendant is entitled to a Franks hearing upon a “substantial showing that the affiant intentionally or recklessly omitted facts required to prevent technically true statements in the affidavit from being misleading.” United States v. Stanert, 762 F.2d 775, 781 (9th Cir.), modified, 769 F.2d 1410 (1985). The issue whether a Franks hearing is required is a mixed question of law and fact reviewed de novo. United States v. Dozier, 826 F.2d 866, 869 (9th Cir.1987). Whitworth argues that the magistrate was purposely misled in two ways. First, he points to paragraph 16 of both affidavits, quoting an unnamed “friend” of Whit-worth’s in Honolulu who said he “has been with Whitworth on many occasions over the years, has discussed stocks and bonds with Whitworth, and has sailed on his boat.” The friend told FBI agents that Whitworth “seemed to have plenty of money” while enlisted in the Navy, that his sailboat had “the latest gadgets,” and that he doubted Whitworth’s explanation that the extra money came from stock deals. Whitworth maintains that he has never owned a sailboat, and that this false statement undermines paragraph 16 and the entire affidavit. The district court, while assuming for purposes of argument that reliance on the Hawaiian informant constituted false statements by the affiant FBI agents, nevertheless concluded that paragraph 16 was not material to the warrant in this case. We agree. Demonstrating that an affidavit contained falsities or omissions is insufficient to require a Franks hearing. Further inquiry must be made to determine if the information was necessary to the magistrate’s issuance of the warrant; that is, whether “the affidavit purged of those falsities and supplemented by the omissions would not be sufficient to support a finding of probable cause.” Stanert, 762 F.2d at 782; see Franks, 438 U.S. at 171-72, 98 S.Ct. at 2684; see also United States v. Burnes, 816 F.2d 1354, 1357 (9th Cir.1987). More troubling, however, is Whitworth’s second claim, that the government’s intentional omission of the search of the mobile home when it applied for a search warrant of the same premises eleven days later precluded a finding of probable cause. Judge Yukasin denied Whitworth’s request for a Franks hearing on this ground in December 1985 following the suppression hearing. A supplemental motion on the same issue was denied in March 1986. Whitworth contends that this court’s decision in Filippelli v. United States, 6 F.2d 121 (9th Cir.1925), mandates a finding that the warrant authorizing the second search of his residence was illegal. In that case, defendant was arrested and convicted for violating the National Prohibition Act. A search warrant for Filippelli’s apartment had been issued and executed on October 25, 1923, based on an affidavit alleging a sale of “jackass” brandy on October 9. A second warrant for the same premises was issued by the same commissioner to the same officers on November 5. It was based on an affidavit charging a sale of wine on October 17. A still and a large quantity of liquor were seized in the second search. Evidence from both searches was introduced at trial over objection. Id. at 122. Filippelli argued, inter alia, that the second search was unlawful because the warrant was based on conduct alleged to have occurred before issuance of the first warrant. The court agreed: We see no defect of form or substance in the first search warrant, or in the affidavit upon which it was based; but, in our opinion, the second search warrant was unauthorized and illegal.... We do not hold or intimate that a second search warrant may not issue in any case after a prior search of the premises, when based on a sale or sales consummated before the first search; but, to authorize a second search, some showing must be made beyond the mere sale of intoxicating liquor antedating the first search. As already stated, a search warrant is authorized for a specific purpose, not as a punishment for crime, and when that purpose has been accomplished the right of search is gone. Courts must presume, in the absence of some showing to the contrary, that officers perform their duties and properly execute the processes placed in their hands.... Id. at 125. Although two warrants were involved in Filippelli while the first search of Whit-worth’s residence was based on consent, he suggests that the 1925 decision prohibits all consecutive searches of the same place absent a showing of intervening probable cause. Under his view, the government’s omission of the first search was material because a magistrate, aware that agents had searched the mobile home eleven days earlier and had already seized evidence of espionage and tax evasion, would not find probable cause to conduct a second search. The government claims that it did not inform the magistrate about the consent search as a matter of strategy because it was afraid of “tainting” the second search. Although it purposely ignored the items previously seized in making its probable cause showing for the warrant, the government reasoned that the mere reference to an earlier search might somehow be found to have been a basis for the magistrate’s decision. In the event that the consent search later was determined to be invalid, the government sought to ensure that the warrant-based search would not be affected. We do not believe it is proper for law enforcement officials to withhold information regarding prior searches of the same premises from magistrates considering warrant applications. If “taint” is feared, the better practice is to advise the magistrate that an earlier consent search had been conducted and provide the reasons why a warrant is still required. The affiant could affirmatively state that nothing obtained in the first search is being relied on in seeking the warrant. At that point, the magistrate can properly evaluate the situation and determine whether probable cause still exists. We acknowledge that the seriousness of the charges in this case, the government’s burden of proof in warrantless search situations, and Whitworth’s initial refusal to consent to the search may have led the government to be specially cautious in this case. Yet it hardly bears repeating that under the careful balance struck by the fourth amendment, “it is the magistrate who must determine independently whether there is probable cause.” Franks, 438 U.S. at 165, 98 S.Ct. at 2681. Notwithstanding our disapproval of the government’s strategy, we agree with the district court that omission of the search did not necessitate a Franks hearing. First, there is no evidentiary conflict to be resolved in a hearing. It is admitted that there was a prior search that was omitted from the affidavit. Second, Whit-worth made no substantial showing that the facts relied on by the district court in determining the materiality of the omission was incorrect. Whitworth basically raises a legal argument that the second search was barred by Filippelli. That case is distinguishable by its own language. The court did not establish a per se rule against consecutive searches in Filippelli, but “presume[d], in the absence of some showing to the contrary, that officers perform their duties and properly execute the processes placed in their hands.” 6 F.2d at 125. The government has argued persuasively that the consent search in this case was not performed as thoroughly as a warrant-based search would have been, so that there still was probable cause for the subsequent search. During the initial search, the agents were working under time constraints and the possibility that Whitworth, who was present and initially had refused permission, would withdraw his consent. As a result, FBI inventory sheets for the first search reveal only twenty entries for items seized. Sheets for the second search, in contrast, contain approximately 220 entries. Moreover, Whitworth admitted at the suppression hearing that he did not believe the consent search was performed thoroughly. On these facts, the failure to disclose the limited consent search could not have affected the decision to issue the warrant. The government’s application was extensive and did not rely on any of the evidence seized earlier. We believe that “the affidavit, once ... supplemented, would provide a magistrate with a substantial basis for concluding that probable cause existed.” Stanert, 762 F.2d at 782. F. The “RUS Letters” Whitworth next argues that the trial court erred by admitting into evidence the four anonymous “RUS letters” received by the FBI during the summer of 1984. It should be noted that his pretrial objection to admissibility appears to have been waived during closing argument when counsel frankly admitted that Whitworth wrote the letters, although questioning their probative value. In any event, we find that the court did not err by admitting the letters. The first letter stated, “I have been involved in espionage for several years.... I didn’t know that the info was being passed to the USSR until after I had been involved a few years.” RUS disclosed various facts about “a significant espionage system,” and offered additional information in exchange for immunity from prosecution. The letters thus were highly probative of the central issue in the case, whether Whitworth participated in Walker’s spy conspiracy. They also constituted the only direct evidence admissible at trial showing that Whitworth was aware he was passing information to the Soviet Union. See infra at 1285. Whitworth claims that the government's offer of proof linking him to the RUS letters did not meet the authentication standard of Fed.R.Evid. 901(a). This rule provides that the “requirement of authentication or identification as a condition precedent to admissibility is satisfied by evidence sufficient to support a finding that the matter in question is what its proponent claims.” Evidence is admissible under Rule 901(a) “once a prima facie case has been made on the issue. At that point the matter is committed to the trier of fact to determine the evidence’s credibility and probative force.” United States v. Johnson, 637 F.2d 1224, 1247 (9th Cir.1980) (citations omitted). Sufficiency of authentication is reviewed for abuse of discretion. United States v. Feldman, 788 F.2d 544, 556 (9th Cir.1986), cert. denied, 479 U.S. 1067, 107 S.Ct. 955, 93 L.Ed.2d 1003 (1987). After reviewing the government’s lengthy offer of proof, Judge Vukasin concluded that a prima facie showing had been made. He found that the espionage conspiracy described in the RUS letters contained numerous points in common with facts known about the Walker Family Spy Ring. The government submitted twenty-three of these comparisons, which were properly considered: “[A] document or telephone conversation may be shown to have emanated from a particular person by virtue of its disclosing knowledge of facts known peculiarly to him.” Fed.R.Evid. 901 advisory committee’s note Example (4). The court’s ruling is supported by other evidence. First, postmarks on the RUS letters corresponded to Whitworth’s location on the days they were mailed. Second, RUS instructed the FBI to respond to his letters in the classified section of Monday editions of the Los Angeles Times. When Whitworth’s mobile home was searched, two copies of the Times were found, and both were Monday editions. Third, it was disclosed that “RUS,” with a single “S,” was the nickname Whitworth used to refer to Russ Salvi, a Navy friend, in memos. Finally, we note that the first RUS letter was written only three weeks after Whit-worth was advised that he would not be paid for material he had delivered to Walker from the U.S.S. Enterprise. Judge Vukasin also was presented with evidence of style similarities between Whitworth’s writings and the RUS letters based on psycholinguistic analysis, but expressly declined to consider it. Whit-worth’s claim that this method is unreliable therefore is irrelevant and we express no view on the matter. Two other arguments against admissibility were properly rejected. The RUS letters, while damaging, did not constitute a “confession,” and no hearing on voluntariness was required. We agree with the court that the “ultimate purpose of the letters ... was not confessional. Rather, they may be viewed as the opening bid in a game which RUS eventually concluded was not worth the candle.” Likewise, the probative value of the letters was not substantially outweighed by the danger of unfair prejudice. Fed.R.Evid. 403. G. Examination of Witnesses The district court made numerous eviden-tiary rulings during the trial on the permissible scope of examination of witnesses and the admissibility of statements. Whit-worth alleges that his sixth amendment right to effective cross-examination was abridged or his case otherwise prejudiced by many of these decisions, especially those involving John Walker. A trial court’s limitation on the scope of cross-examination and its evidentiary rulings are reviewed for abuse of discretion. United States v. Poschwatta, 829 F.2d 1477, 1481, 1483 (9th Cir.1987), cert. denied, — U.S. —, 108 S.Ct. 1024, 98 L.Ed.2d 989 (1988). We cannot merely substitute our judgment for that of the district court to reverse under this standard, but must have a definite and firm conviction that the court committed a clear error of judgment in the conclusion it reached upon consideration of the relevant factors. See United States v. Ruffen, 780 F.2d 1493, 1495 (9th Cir.), cert. denied, 479 U.S. 963, 107 S.Ct. 462, 93 L.Ed.2d 407 (1986). The government’s case against Whit-worth centered around the testimony of Walker, his coconspirator and leader of the espionage ring. Walker described with some detail a series of twenty-three meetings he and Whitworth had between February 1975 and April 1984. Whitworth received a total of $332,000 in exchange for cryptographic material and other classified Navy information, which Walker relayed to the Soviet Union. Given that Walker’s credibility was essential to the government’s case, Whitworth contends that the district court should have allowed the broadest possible cross-examination. The government insists that this in fact was done, and any legitimate damage that Whitworth intended to do to Walker’s credibility was accomplished. “This court has repeatedly expressed its concern that defense counsel be given maximum opportunity to impeach the credibility of key government witnesses.” United States v. Williams, 668 F.2d 1064, 1070 (9th Cir.1981). The right to cross-examination is not absolute, however. On the contrary, trial judges retain wide latitude insofar as the Confrontation Clause is concerned to impose reasonable limits on such cross-examination based on concerns about, among other things, harassment, prejudice, confusion of the issues, the witness’ safety, or interrogation that is repetitive or only marginally relevant. And as we observed earlier this Term, “the Confrontation Clause guarantees an opportunity for effective cross-examination, not cross-examination that is effective in whatever way, and to whatever extent, the defense might wish.” Delaware v. Fensterer, 474 U.S. 15, 20 [106 S.Ct. 292, 295, 88 L.Ed.2d 15] (1985) (per curiam) (emphasis in original). Delaware v. Van Arsdall, 475 U.S. 673, 679, 106 S.Ct. 1431, 1435-36, 89 L.Ed.2d 674 (1986). Whitworth cites three examples of undue limitation on cross-examination. First, inquiry was not permitted into whether Walker asked his wife, Barbara, to engage in prostitution to pay debts. The court found this to be a specific instance of conduct that was irrelevant to untruthfulness and therefore inadmissible under Fed.R. Evid. 608(b). The court also sustained on relevancy grounds an objection to a followup question regarding Barbara’s participation in a drop in 1971. Finally, while cross-examination was permitted regarding Walker’s membership in the Klu Klux Klan, Whitworth was not allowed to inquire into Walker’s racial views or enter a photograph of him in Klan regalia into evidence. The court found the evidence to be irrelevant or, if relevant, to be unfairly prejudicial under Fed.R.Evid. 403. We do not believe that Judge Yukasin abused his discretion. The record indicates that Walker was subjected to extensive cross-examination and recross-examination over a four-day period. During trial, evidence of his seventeen-year espionage career, the recruiting of his family and best friend to spy, and his plea agreement was fully explored. Inquiry also was permitted into alleged marital infidelity and Walker’s purported suggestion that his daughter, Laura, get an abortion so that she could remain in the Army and begin spying for him. In short, the jury was given ample opportunity to despise Walker and discredit his testimony. See Batchelor v. Cupp, 693 F.2d 859, 865 (9th Cir.1982), cert. denied, 463 U.S. 1212, 103 S.Ct. 3547, 77 L.Ed.2d 1395 (1983). On a related matter, Whitworth claims that numerous irrelevant or speculative statements by Walker were improperly admitted at trial. One allegation requires our attention. During redirect examination, Walker was asked whether he ever had “suspicions or apprehensions that Mr. Whitworth knew that the real buyers were the Soviets?” After a defense objection was overruled, Walker answered affirmatively. He stated that his belief was based on “[c]ommon sense, sir. It made no sense that a friendly government or a criminal element would buy cryptographic material.” The question clearly asked Walker to speculate about Whitworth’s state of mind. The government argues, however, that the door to exploring Walker’s “apprehensions or suspicions” was opened by Whitworth’s counsel on cross-examination. It refers to a colloquy where Walker was asked whether he thought that Whit-worth’s delivery of certain poor-quality information in late 1981 and in 1983 was intentional: Q: At this point in time, did you begin to develop any apprehensions or suspicions that he might be pulling your leg a little bit or stringing you along? A: I had minor suspicions, yes. * sic * * * * Q: Did you begin to suspect or question any further during this period what was going on in his head with respect to your relationship? A: Yes, I did. Q: What precisely went through your mind at that point, if you recall? A: I was concerned that he was attempting to pull the classic ripoff or blackmail the Soviets by saving this stack of secret messages. Under the rule of curative admissibility, or the “opening the door” doctrine, the introduction of inadmissible evidence by one party allows an opponent, in the court’s discretion, to introduce evidence on the same issue to rebut any false impression that might have resulted from the earlier admission. See United States v. Segall, 833 F.2d 144, 148 (9th Cir.1987); United States v. Makhlouta, 790 F.2d 1400, 1402-03 (9th Cir.1986); I J. Wigmore, Evidence § 15 (Tillers rev. 1983). The doctrine does not permit the introduction of evidence that is related to a different issue or is irrelevant to the evidence previously admitted. McCormick on Evidence § 57 (3d ed. 1984). The district court abused its discretion in admitting the statement. On cross-examination, Whitworth’s counsel asked Walker whether he had suspicions regarding Whit-worth’s sincerity after the poor deliveries. This called for an answer within Walker’s personal knowledge. The question on redirect, in contrast, related to what Walker believed Whitworth knew about the buyer’s identity. The door was never opened for such an inquiry. This testimony should not have been presented to the jury. We nevertheless conclude that the error was harmless and a new trial is not required. The admission of Walker’s opinion evidence in this case did not amount to a constitutional error. See United States v. Cox, 633 F.2d 871, 876 (9th Cir.1980), cert. denied, 454 U.S. 844, 102 S.Ct. 159, 70 L.Ed.2d 130 (1981). When a nonconstitutional error is involved, a defendant’s conviction will be reversed only if it is more probable than not that it materially affected the verdict. United States v. Rhodes, 713 F.2d 463, 475 (9th Cir.), cert. denied, 464 U.S. 1012, 104 S.Ct. 535, 78 L.Ed.2d 715 (1983). Whitworth’s defense to the espionage charges was based on a lack of knowledge that the classified information he supplied to Walker was being relayed to the Soviet Union. Walker’s challenged statement, while contradicting this theory, could have had little effect on the verdict because it was merely cumulative. Standing alone, the first RUS letter supplied the jury with overwhelming direct evidence that Whit-worth knew the identity of the buyers. Although Walker testified that he never told Whitworth that they were dealing with the Soviet Union, the jury could have inferred Whitworth’s knowledge from the surrounding circumstances. For example, Walker testified that he and Whitworth agreed as a “cover story,” in the event that they were arrested, “to explain that the material was being sold to a friendly nation or ally.” In addition, Walker stated that Whitworth often asked about Soviet spies that had been caught, and requested that Walker find out “from the buyers of our material” what they had done wrong. As a final evidentiary matter, Whitworth alleges additional errors by the district court regarding the examination of various witnesses. He contends that cross-examination of government witness Michael O’Connor and direct examination of defense witness Laurie Robinson were improperly restricted. Whitworth also argues that government witnesses Gerald Richards, Arthur Walker, Lynn McShane, Jennifer Anderson, Beverly Gansberger, and John Barron were allowed to testify to irrelevant matters. Upon a review of the record, we find that the court did not abuse its discretion in its rulings regarding these witnesses. H. Sufficiency of the Evidence Whitworth challenges the sufficiency of the evidence on the espionage charges. He argues that the government failed to prove beyond a reasonable doubt that he knew the information was going to the Soviet Union. A criminal conviction is