U.S. officials are investigating the possibility that physicist Wen Ho Lee copied nuclear secrets from the classified computer system at Los Alamos National Laboratory onto computer tapes to help his native Taiwan.

Although the initial investigation of the 60-year-old scientist focused on his contacts with nuclear researchers in China, investigators have expanded their attention to include Taiwan as well as the communist mainland because of Lee's long association with the island and his consulting work for a military institute there.

Lee has not been charged with passing secrets to any country, and authorities have said they do not have sufficient evidence to charge him with espionage. But a federal judge this week ordered him to be held without bail on a 59-count indictment alleging that he downloaded a huge trove of nuclear weapons data onto 10 tapes, seven of which are missing.

Lee's defense lawyers have said the seven tapes were destroyed, but they have not offered any proof. Nor have they explained why he copied the highly classified information in violation of security regulations.

Experts testified in court this week that if the tapes fell into the hands of a foreign country, they would allow that nation to design a reliable nuclear weapon quickly, even without testing.

Investigators said they still believe that Lee may have passed secrets to China, which has possessed nuclear weapons since 1964. But Taiwan is considered by U.S. intelligence agencies to be a potential nuclear weapons state. On at least two occasions in the past 20 years, the United States has pressed the Taiwanese to shut down clandestinely built reprocessing facilities designed to produce weapons-grade plutonium.

Taiwan denies that it is developing nuclear weapons but acknowledges having an extensive program of peaceful research on nuclear energy. U.S. officials believe that at least some Taiwanese officials would like to have a nuclear deterrent against invasion from the People's Republic of China.

The most direct connection between Lee and Taiwan's research effort came in the spring of 1998, when he traveled to the island to work for several weeks as a consultant at the Chung Shan Institute of Science and Technology, a top-secret, military-run facility devoted to nuclear research and missile development.

Last December, Lee returned to Taiwan for three weeks, delivering a speech at Chung Shan and again consulting with scientists there. During that visit, according to court testimony by a Los Alamos official, Lee dialed up the main computer at the national laboratory and used his password to gain access to the classified nuclear files he had previously downloaded.

Although Lee's 1998 trips to Chung Shan were approved by his Los Alamos group leader, Lee's departure from this country was a surprise not only to the FBI but also to the Energy Department's new counterintelligence chief, Edward J. Curran, who was monitoring the espionage investigation then underway.

On his return, Lee took a polygraph test in which he was asked whether he had committed espionage or divulged secret information. According to his attorneys, he passed.

Born on Taiwan, Lee came to the United States in 1964; he has two sisters who still live on the island. According to investigators, he began his open, legal assistance to Taiwan's nuclear research program in the late 1970s. By that time, thanks to the Nixon administration's normalization of relations with Beijing, the number of U.S. troops on Taiwan had been sharply reduced, all American nuclear weapons had been removed, and Taiwan was under the U.S. nuclear umbrella.

"Lee did not hide his support for Taiwan," one of the investigators said. According to a former colleague at Los Alamos, he was an active supporter of the Taiwanese independence movement.

In testimony last June before a closed Senate Judiciary Committee hearing, Attorney General Janet Reno outlined Lee's assistance over the past 20 years to Taiwan's nuclear research program.

Reno said Lee told FBI interviewers in 1983 that "he had been in contact with Taiwanese nuclear researchers since 1977 or 1978, and had done consulting work for them in addition to giving them unclassified research papers."

"Starting in 1980," Reno said, "he would receive requests for papers and reports from the Taiwanese Embassy, which he would then copy and mail to the embassy."

In 1984, Lee was polygraphed by the FBI after he was heard on a wiretap making a telephone call to a Taiwan-born former scientist at Lawrence Livermore National Laboratory who had been forced to resign after allegedly trying to aid mainland China. Lee showed deception on the so-called lie detector test, and he later admitted to the FBI that he had been giving Taiwanese officials information that was unclassified but not generally available.

"Lee thought that this other scientist was in trouble for doing the same thing that Lee had been doing for Taiwan, and thus Lee had become concerned," Reno said.

In 1992, Lee and his wife made a pleasure trip to Taiwan and Hong Kong. In 1993 and 1994, Lee was informed that his job was "at risk" because of pending cutbacks in employment at Los Alamos. It was at this time that he began meticulously downloading huge computer files, first to his own unsecured office computer and later to portable tapes.

During this same period, Lee sent out seven letters expressing interest in jobs at research centers overseas, according to government investigators. Two of the letters went to institutes in Taiwan, but not to the military-run Chung Shan.

Lee's attorney, Mark Holscher, said yesterday that Reno's testimony shows "Doctor Lee had proper, lawful contacts with Taiwan" and "directly undercuts the Department of Justice's attempts to somehow link Doctor Lee with the country [mainland China] that is an avowed enemy of Taiwan."

James Lilley, a former U.S. ambassador to China and previously a senior CIA official, said yesterday he would be "very suspicious" of any notion that Lee was working for Taiwan. "My instinct is it could be a diversionary use of Lee by the Chinese, setting him up with the opposition, just like getting his wife working with the FBI."

Lilley noted that he does not have access to any of the classified material in the case. But "my sense is, Taiwan is sort of a red herring," he said.

Staff writers Steven Mufson and Vernon Loeb contributed to this report.