Calling Wen Ho Lee an "unprecedented" security risk to the United States, government prosecutors asserted today in U.S. District Court here that the former physicist at Los Alamos National Laboratory cannot account for top-secret nuclear weapons design information downloaded from the lab's computers and should remain in jail until his trial begins.

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 to any unauthorized parties.

But U.S. Attorney John J. Kelly wants Lee to remain in jail pending trial for what could be a year or more, arguing that his inability to account for seven portable computer tapes containing enough information to build a thermonuclear weapon, coupled with a pattern of deception at Los Alamos, shows intent to damage the nation's security.

"When I first realized what was downloaded by Dr. Lee, I realized I was looking at a chilling collection of codes and files," Richard Krajcik, deputy director of Los Alamos's secret X Division, told a packed courtroom. "It really represents a capacity someone could use to design and analyze [U.S. nuclear] weapons."

Lee was indicted Dec. 10 on 59 felony counts of mishandling classified information downloaded in 1993, 1994 and 1997 from Los Alamos's secure computer system. He is accused of transferring nearly all of the files to 10 computer tapes. The government says seven of the tapes cannot be accounted for; Lee's attorneys say the seven tapes have all been destroyed, although they have not provided any evidence to support that claim.

Thirty-nine of the 59 felony counts involve violations of secrecy provisions in the Atomic Energy Act and carry maximum sentences of life in prison. But to make those charges stick, the government must prove that Lee willfully intended to damage national security.

Lee was fired from his post in the lab's X Division in March for allegedly tampering with classified documents and failing to report foreign contacts that occurred in the mid-1980s during trips he made to China. At the time of his firing, Lee was identified by federal authorities as their prime suspect in a Chinese espionage investigation centering on evidence that China may have stolen design information related to the W-88 warhead, America's most sophisticated thermonuclear weapon.

But federal authorities have since acknowledged that they do not now have any evidence showing Lee spied for China or any other foreign government. Kelly, in fact, has noted in court papers filed here that the charges brought against Lee for mishandling classified information are unrelated to the earlier espionage probe, which has since been widened to include numerous other nuclear weapons facilities. Lee's downloading of classified material was not discovered by investigators until late last March, weeks after he was fired and publicly identified as an espionage suspect.

Lee's supporters--several dozen of whom gathered here today outside the courtroom in a show of support for the imprisoned scientist--say the cases are linked. The government never would have even searched Lee's computer and discovered the downloaded information if federal officials hadn't improperly singled him out as a potential spy, at least partly on the basis of his ethnicity as a Chinese American, they say.

Like the broader case itself, the bail hearing that began today is expected to center on the question of whether Lee intended to harm the nation in downloading secret data, a key consideration in whether he can be allowed out of jail on bail without further threatening the nation.

Krajcik called the data downloaded by Lee "the crown jewels" of America's nuclear weapons program and said Lee's "private collection" of weapons codes existed nowhere else in the country except Los Alamos and Lawrence Livermore national laboratories.

Krajcik's testimony came after that of Cheryl Wampler, a Los Alamos computer official who testified for more than two hours about Lee's methodical downloading of data from classified to unclassified laboratory networks and then onto unsecured portable computer tapes.

Wampler testified that Lee changed the coding on every file he downloaded from classified to unclassified, even though lab security rules strictly prohibit any secret material from being removed from the secure system. Once the highly sensitive nuclear weapons codes--which simulate nuclear explosions and can be used to reverse-engineer U.S. warheads--had been improperly downloaded, Lee copied them onto tapes using a machine in an unsecure section of the laboratory.

That machine, located in Los Alamos's T Division, "was imminently available to not very sophisticated hackers from the Internet" in addition to as many as 10 unclassified users working in that section of the lab, Wampler testified. She noted, in response to a question from Judge Parker, that there is no evidence any hacker or unauthorized users ever accessed Lee's surreptitiously downloaded files.

During cross-examination, one of Lee's attorneys, John Cline, repeatedly attempted to show that Lee did not take many other steps in downloading the data that would have covered his tracks.

Cline pointed out that, as he moved the files from the classified system, Lee retained file names that showed that the data included nuclear weapons codes. He also said that his client was fully aware that every move he was making on the computer network was being recorded and that his chosen password--WHL--would show up in security logs in both the classified and unclassified systems.

And once Lee had copied the downloaded files onto tapes, wouldn't it have made sense for him to delete the files from his unsecure computer, instead of leaving them there for government investigators to find six years later? Cline asked.

"It would have made sense, yes," Wampler said.

CAPTION: Supporters of fired Los Alamos scientist Wen Ho Lee gather at the federal courthouse in Albuquerque.