A top nuclear weapons designer at Los Alamos National Laboratory testified today in U.S. District Court that physicist Wen Ho Lee told him in a brief conversation last February that he may have inadvertently passed classified nuclear weapons data to a foreign country.

But the weapons designer, Richard Krajcik, conceded under cross-examination that he might have misunderstood Lee. He also acknowledged that he and Lee later argued over the possibility that classified weapons information was inadvertently contained in scientific articles written by Lee that were reviewed and approved for publication by his superiors at Los Alamos.

Krajcik's dramatic testimony represented the first time any U.S. official has publicly offered evidence that Lee may have passed classified information to another country since the former Los Alamos physicist was fired in March for security violations and identified by government officials as an espionage suspect.

Krajcik testified that Lee came to his office late one afternoon in February, having just been informed that he had failed an FBI polygraph investigation into whether he failed to report contacts with Chinese scientists.

"He then indicated to me that while he did not intentionally pass on [classified] information to a foreign country, he may have accidentally passed [classified] information to a foreign country," Krajcik said.

His testimony was attacked during cross-examination by Lee's lead defense attorney, Mark Holscher, who established through a series of questions that Krajcik failed to mention Lee's remark to FBI agents for five months, even though he was assisting them in a criminal investigation into China's apparent theft of design secrets related to the W-88, America's most sophisticated thermonuclear weapon.

Holscher pointed out that Krajcik, assisting FBI agents, argued with Lee during an all-day interrogation on March 5 about whether articles Lee had written with lab approval inadvertently disclosed classified information. Holscher also released copies of a Department of Energy polygraph examination that Lee passed on Dec. 23, 1998, in which he was asked whether he had ever committed espionage, inadvertently passed information to China or associated with anyone who committed espionage.

Holscher said Lee's scores, determined by three separate department polygraph reviewers, "were off-the-charts truthful."

As for Lee's truthfulness during the FBI-administered polygraph in February, FBI agent Robert A. Messemer took the witness stand after Krajcik and testified that Lee was deceptive when asked about his failure to report contacts with Chinese scientists during approved trips he made to Beijing in 1986 and 1988.

In describing Lee's polygraph results, Messemer revealed for the first time that Lee, during the polygraph examination, admitted helping a Chinese nuclear weapons scientist solve a problem during his 1986 trip. Messemer also testified that Lee, having admitted during his Department of Energy polygraph to having a clandestine hotel meeting with Chinese officials in 1988, subsequently told FBI examiners that he also provided assistance during that trip to a Chinese scientist.

The court recessed tonight before Messemer could be cross-examined by Lee's attorneys.

Testimony from Krajcik and Messemer came during the second day of a bail hearing requested by Lee's attorneys. Prosecutors are trying to keep Lee in jail for up to a year while he awaits trial on 59 felony counts of mishandling classified information.

U.S. Attorney John J. Kelly has argued in court papers that Lee's inability to account for computer tapes containing nuclear secrets sufficient to build a thermonuclear weapon, coupled with a pattern of deception at Los Alamos, makes his release on bail a threat to national security.

Lee's attorneys have asked Judge James A. Parker to revoke a magistrate's Dec. 13 denial of bail, arguing that an exhaustive FBI investigation has failed to turn up any evidence that the 59-year-old scientist revealed--or intended to reveal--any of the classified information he downloaded from Los Alamos's secure computer system to any unauthorized parties.

Lee was indicted Dec. 10 on 59 felony counts of mishandling classified information downloaded in 1993 and 1994. He is accused of transferring nearly all of the computer files--the equivalent of more than 800,000 pages of information--to 10 computer tapes. Prosecutors say seven tapes are missing.

Lee's attorneys say the tapes have been destroyed, asserting that Lee began deleting numerous classified files in his possession after he was transferred out of Los Alamos's top secret X Division in December 1998 and realized he was no longer cleared for such access.

While U.S. authorities identified Lee as a suspect in the Chinese espionage case at the time of his firing in March, they have subsequently acknowledged that they do not now have any evidence showing Lee spied for China or any other foreign government.

Lee's downloading of classified material was not discovered by investigators until late March, weeks after he was fired and publicly identified as an espionage suspect, and Kelly has said in court papers filed here that the charges brought against Lee for downloading nuclear weapons data are unrelated to the earlier espionage probe.

Prosecutors offered Krajcik's testimony about his brief conversation with Lee in February not to support espionage charges but to to bolster their contention that Lee remains a national security threat who should be denied bail.

Krajcik testified that Lee downloaded the "crown jewels" of America's nuclear weapons program from a secure to an unsecure network, which "increases the risk to 270 million Americans. . . . The specific risk is that [unauthorized users] could develop nuclear weapons that they wouldn't have been able to produce before."

But in cross-examining Krajcik and another Los Alamos official, Lee's attorneys have asserted that Lee never took a number of obvious steps, such as changing file names and deleting files, that would have made it harder for security officials to track his actions--steps that would have been taken, they said, by a spy or an avowed enemy of the United States.

Lee's lawyers have also attacked the government's claim that Lee surreptitiously set out to "cover his tracks" by deleting classified files after it became clear to him in January and February that he was under investigation for espionage. Under cross-examination, Los Alamos computer official Cheryl Wampler acknowledged that Lee called Los Alamos's computer help desk in January, three days after he was interviewed by FBI agents, and asked for help in deleting classified files.

At the close of testimony Monday, Parker asked Krajcik what harm it would do to let Lee out of jail on bail.

"If he were available to talk to people about the codes," Krajcik said, "he could pass on valuable information to people about [how to use] the codes."

To which Parker, perhaps offering a glimpse of how he was leaning on the question of bail, replied: "So it comes down to restricting his ability to communicate with others."