Former Los Alamos scientist Wen Ho Lee has told government investigators that he cannot account for several computer diskettes onto which he downloaded nuclear secrets, according to Clinton administration officials.

"At least two high-density [diskettes] that he purchased are missing," said one federal official familiar with the FBI investigation.

FBI Director Louis J. Freeh briefed some members of Congress about the case this week, saying he expects Lee to be indicted for gross negligence in handling classified information. The transfer of some of that information to computer diskettes, which are now missing, "increases the magnitude of the violation," the official said.

A congressional source said yesterday that there are indications that Lee copied nuclear weapons computer codes onto the diskettes, some of which have been retrieved by federal authorities. But Lee has failed to turn over--and says he does not know the whereabouts of--other diskettes that could contain millions of lines of computer code, the source said.

Lee, a Taiwan-born U.S. citizen, was fired in March from his job at Los Alamos's top-secret X Division for various security violations, including failing to report contacts with foreign scientists. He also showed deception on an FBI polygraph test.

At the time of his firing, he was identified as the government's prime suspect in China's alleged theft of classified information on America's most advanced thermonuclear warhead, the W-88. U.S. officials subsequently acknowledged that they have no hard evidence that he passed secrets to China, but they have continued to build a criminal case against him for downloading classified information to his unsecured office computer.

The downloading came to light only after Lee's firing, when he permitted investigators to look through his computer. He has maintained, however, that his multiple passwords guaranteed that unauthorized persons could not get access to the material.

Investigators also searched Lee's home and uncovered other classified information, according to government officials.

Lee's attorney, Mark Holscher, said last night that the government continues to leak false and misleading information about Lee without evidence that any of the computer files ended up in the hands of unauthorized parties.

"One should not forget that the leaks of six months ago falsely branded Dr. Lee as a spy," Holscher said. "We are facing a moving target."

Holscher added that the FBI has refused to give Lee an inventory of diskettes that were allegedly in his possession. Claiming now that he cannot account for diskettes, Holscher said, "is the most recent spin on events. It is impossible to disprove a negative."

The lawyer also suggested that U.S. officials may have leaked information about the diskettes to put Lee back on the defensive after it became known this week that the FBI has turned up evidence suggesting that China may have obtained nuclear secrets from facilities other than Los Alamos.

The FBI, which widened its probe in September, traced errors in a Chinese military document to one of the "integrators" that put together the W-88, officials said.

A Defense Department official explained that the "integration" process for the W-88 involves the Energy Department's national labs, which design and build nuclear warheads; Lockheed Martin Corp., which manufactures missiles and nose cones; and the Navy, which serves as the overall integrator.

Rod Geer, a spokesman for Sandia National Laboratories, said the lab has worked closely with federal investigators since the espionage investigation began four years ago.

"Since we are responsible for the design of the non-nuclear components of the W-88 and all other warheads, it's logical that they would want to talk to us," Geer said. "We are not under investigation ourselves, and we have not been implicated in any wrongdoing."

A spokesman for Lockheed Martin said the firm has no corporate involvement in the espionage probe. He said Lockheed is cooperating with federal investigators "in our role as manager of significant portions of DOE's weapons complex."

Sen. Arlen Specter (R-Pa.), who is investigating the Clinton administration's handling of Chinese espionage as chairman of the Judiciary Committee's subcommittee on government oversight, said he still isn't sure the FBI is on the right track.

"It's about as hard to figure out what the bureau has done as it is to figure out what Wen Ho Lee has done," Specter said.