The FBI has found new evidence suggesting that China may have stolen information about the most advanced U.S. nuclear warhead from one of the weapon's assemblers, widening an investigation once focused almost exclusively on Los Alamos National Laboratory and one of its staff scientists, Wen Ho Lee.

The evidence emerged after weapons scientists at Los Alamos noted errors in a Chinese intelligence document that sparked the initial FBI and congressional investigations into Los Alamos and Lee. The telltale errors, contained in a description of the miniaturized W-88 warhead, were traced to one of the contractors and defense installations that assemble nuclear weapons, government sources said.

While the new evidence does not completely eliminate Los Alamos or Lee, the sources said, it indicates that the most likely origin of the information is one of the weapons "integrators." These include Sandia National Laboratories, which puts together prototypes of some warheads; Lockheed Martin Corp., which attaches warheads to missiles; and the Navy, which supervises the process.

One source said the analysis "widened the circle and gave convincing evidence" backing up the contention, long voiced by scientists at Los Alamos and officials at the Department of Energy, that China could have obtained classified information about the W-88 and other U.S. nuclear warheads from any of dozens of facilities.

A Lockheed Martin spokesman said yesterday the company "is cooperating with the government in its investigation and is not under investigation nor implicated in any wrongdoing."

Attorney General Janet Reno and FBI Director Louis J. Freeh told Congress in September that they were starting their investigation into Chinese espionage over again and assigning scores of additional agents to broaden the probe.

The decision to go back to square one, they explained, came after they concluded that the initial inquiry was botched by FBI agents and Department of Energy intelligence officials who focused prematurely on Lee, a Chinese American physicist who worked for almost 20 years at Los Alamos's top secret X Division.

Lee was fired by Department of Energy officials in March for violating lab security procedures, and he was identified as the government's prime espionage suspect. U.S. officials now say it is likely that Lee soon will be indicted for gross negligence in handling classified information by transferring top-secret computer programs to his unsecure desktop computer.

But the officials acknowledge that the espionage case against Lee was circumstantial and that they do not have evidence he turned over nuclear secrets to China. Together with a dire report on Chinese espionage by a House select committee headed by Rep. Christopher Cox (R-Calif.), the FBI's espionage probe at Los Alamos created a political furor earlier this year.

Cracks in the case began to appear in June, when the President's Foreign Intelligence Advisory Board criticized the Department of Energy and the FBI for focusing almost exclusively on Lee when there was no hard evidence that he, or anyone else at Los Alamos, was the source of classified information somehow obtained by China.

Then, in August, the former chief of counterintelligence at Los Alamos publicly alleged that federal investigators had improperly targeted Lee because of his ethnicity.

Lee's attorney, Mark Holscher, said yesterday that the FBI's new evidence "is further proof that the focus of the investigation on Doctor Lee was inappropriate and that to continue to prosecute him for lesser charges is unfair."

The espionage investigation at Los Alamos was triggered in late 1995 by a classified Chinese military report on nuclear weapons that was obtained by the CIA in Taiwan. The document, dated 1988, was provided by a Chinese official who offered to spy for the United States and who, over a period of time, also turned over hundreds of other papers.

Because the Chinese official volunteered and was not recruited, he is known in intelligence parlance as a "walk-in," and the military document he provided has become known as the "walk-in document." It drew immediate interest because it discussed the need for Beijing to design a new intercontinental missile and cited the W-88 as an example of what Chinese scientists should be developing.

Among the details in the walk-in document were the dimensions and shape of the primary nuclear element in the W-88, which were configured to reduce the size of the warhead.

Energy Department intelligence officials, who believed they had seen some changes in the nuclear devices China had begun testing in the early 1990s, brought in experts to analyze the document. They decided that some of the information was classified and must have been obtained by espionage.

Helped by the FBI, an Energy Department team attempted to determine the origin of the leak. It quickly focused on Los Alamos, where the W-88 was designed and developed.

From the start, however, the Chinese document's description of the radius of the W-88's nuclear trigger was less precise than would be needed to construct such a device, according to one U.S. nuclear scientist familiar with the document. "The information was not detailed enough to be accurate," the scientist said.

Although officials yesterday would not discuss the particular errors involved in the FBI investigation, the measurement of the trigger was said to be the kind of data under scrutiny.

At one point in late 1996, the CIA determined that the Chinese official who delivered the document was not to be trusted and probably was a double agent. The agency also warned that the documents he had provided may have been approved for delivery by Chinese intelligence and, therefore, were suspect.

That finding held up the Energy Department and FBI investigators for several months. In the end, however, they decided that enough of the information was classified to justify an inquiry into how China obtained it.