Late last week, the FBI arrested Wen Ho Lee, a Chinese American scientist at Los Alamos National Laboratory, on charges of mishandling nuclear secrets. But the FBI's probe of alleged nuclear espionage is far from over.

Lee, who was once at the center of the investigation, is one of many possible suspects. Although the felony charges against him could carry a life sentence, federal prosecutors concede there is no evidence that he passed secrets to China.

The FBI is now looking elsewhere--at other national laboratories, defense plants and Navy missile contract administrators--for the source of information allegedly stolen by China about the W-88, America's most advanced nuclear warhead. Yet the bureau's former chief analyst for Chinese counterintelligence doubts that its investigators ever will find a master spy.

Why? Because China rarely recruits master spies, according to Paul D. Moore, who spent 20 years following operatives from China's premier intelligence service, the Ministry of State Security.

Moore and other experts believe that China methodically sifts information from open sources and combines it with many small leaks of secret information, typically culled one at a time from American scientists visiting China, a strategy very different from the Cold War model of moles and master spies, smuggled microfilm and secret meetings in the park.

"If you're meeting with someone who is a spy in the park, you might be seen," Moore said. "But if you're a scientist authorized to discuss three things with the Chinese and you discuss four things, how is the government going to know about that?"

Attorney General Janet Reno and FBI Director Louis J. Freeh told Congress in September that they were beginning a new and much broader inquiry into Chinese espionage after concluding that FBI agents and Department of Energy investigators botched the case three years ago by focusing prematurely on Lee--a Taiwan-born U.S. citizen who worked for nearly 20 years at Los Alamos's top secret X Division--and ignoring other potential leads.

Lee was fired from his job in March for security violations and was identified as the government's prime suspect after he failed a polygraph test. He has admitted that he transferred top-secret information from the classified computer system at Los Alamos to his unsecure desktop computer, but he claims he was just trying to protect the information from computer crashes.

Moore believes the FBI erred, not so much by targeting Lee, but by assuming that there had to be a spy inside the nation's nuclear weapons complex, based on fragments of classified information contained in a document handed over to the CIA by a Chinese "walk-in" agent.

"So far as I can see, most folks are simply asking the wrong question: Who stole what secrets?" Moore said. "That's always an important question, but the really critical one is: How and where were these thefts accomplished?"

Moore believes that China's intelligence agency targets Chinese Americans to the exclusion of others--heavy drinkers, disgruntled employees, ideologues--who might seem to be better targets.

But China usually doesn't try to pry information out of Chinese Americans in the United States in ways that would leave much evidence for investigators to uncover, Moore said. And China almost never tries to obtain large quantities of secrets from one person, the way the KGB once did and the CIA still does, he said.

China's track record with Chinese Americans, Moore said, is extremely poor; most Chinese Americans refuse to cooperate with intelligence agents. But China ultimately succeeds, he said, because it employs "a kind of junk mail approach to intelligence," trying over and over again with Chinese Americans and other official visitors to China. They succeed often enough in forcing "good people to do bad things"--sometimes without fully realizing it--to make Chinese espionage effective, in part because it's so difficult to detect.

The price China pays for this thousand-grains-of-sand approach, Moore said, is that it takes years to assemble the same amount of information a master spy could stash at a dead drop in an hour.

As for the wisdom of the FBI's expanded reinvestigation, Moore said he understands why Freeh ordered his agents back to square one in their search for a Chinese spy. "The cat is out of the bag and people expect you to do certain things," Moore said. "But it doesn't make sense to expect that they're going to come up with this magic answer."

Further complicating the investigation is the so-called walk-in document that triggered the probe. The 74-page document is a classic illustration of China's methodical collection techniques, apparently gathered in bits and pieces from many public and private sources.

It contains dozens of facts about U.S. nuclear warheads, most displayed in a two-page chart. On one side are various Air Force and Navy weapons, including some older bombs as well as the W-80 warhead for cruise missiles, the W-87 for the Minuteman III intercontinental ballistic missile and the W-88 for the Trident II. Along with each warhead in the chart are details on size, weight, explosive yield, reentry vehicle (including a line sketch), range and accuracy.

Some of this information is correct, and some is not, according to a senior administration official. He noted that part of the information included in the chart was publicly available in 1988, when the document purportedly was written.

Scientists at Los Alamos, recently asked by the FBI to look again at the walk-in document, spotted some errors or imprecisions in the W-88's classified dimensions and traced them to the warhead's assemblers, the first evidence suggesting a source outside Los Alamos, where the miniaturized warhead was designed.

Others are now puzzling over the document's additional errors. Can they be traced to a particular source? Or were they included by the Chinese for a reason?

A former senior Pentagon officer said some errors may have resulted because the information was translated from English to Chinese, then translated back again by U.S. intelligence.

Most confounding of all, however, is the CIA's conclusion that the walk-in agent who delivered it in Taiwan was a double agent still under China's control. Early on, the CIA warned Energy Department officials and the FBI, who had already begun to focus on Wen Ho Lee as the alleged source, that the document's contents were questionable.

As a result, the FBI halted its inquiry for several months in late 1996. It resumed, according to one participant, after Notra Trulock, the Energy Department's former director of intelligence, decided that China's source of classified information had to be uncovered, regardless of whether the document had been planted by Chinese intelligence.

Thus, the question remains: Why would China hand U.S. intelligence a document describing what Beijing knew about U.S. nuclear warheads?

Some analysts think the walk-in agent initially delivered genuine materials because he was not, at first, under Beijing's control. The quality changed after the first deliveries, one source said, raising questions about the agent's reliability.

"There is no consensus," one senior administration official said last week.

Recently, an Energy Department official said some intelligence analysts have seen the entire exercise as a "disinformation operation" designed "to test our reactions and throw us off the scent."

Under this scenario, the official said, the Chinese have very accurate information about U.S. weapons, "but they want to have us assume their intelligence is not very good."

A more sinister twist on this approach is that the Chinese have one or more excellent sources of information on the U.S. nuclear program and offered up the document to lead U.S. investigators down a different path. Still another interpretation, voiced by one former Department of Energy official, is far more direct: China delivered the document to Taiwan, the source said, to intimidate Taiwan.

CAPTION: Former Los Alamos physicist Wen Ho Lee is taken into custody Friday after indictment.