The Administration explained aspects of the case to aides working for the House and Senate intelligence committees beginning in 1996. But few in Congress grasped the magnitude of what had happened.

In July 1998, the House Intelligence Committee requested an update on the case, officials said. Mr. Trulock forwarded the request in a memo to and in conversations with Elizabeth Moler, then Acting Energy Secretary. Ms. Moler ordered him not to brief the House panel for fear that the information would be used to attack the President's China policy, according to an account he later gave Congressional investigators.

Ms. Moler, now a Washington lawyer, says she does not remember the request to allow Mr. Trulock to brief Congress and denies delaying the process.

Key lawmakers began to learn about the extent of the Chinese theft of American nuclear secrets late in 1998, when a select committee investigating the transfers of sensitive American technology to China, headed by Representative Christopher Cox, Republican of California. heard from Mr. Trulock.

Administration officials say that Congress was adequately informed, but leading Democrats and Republicans disagree. Norman Dicks, Democrat of Washington, the ranking minority member on the House intelligence panel and also a member of the Cox committee, said that he and Porter Goss, Republican of Florida, who heads the intelligence panel, had not been clearly informed.

''Porter Goss and I were not properly briefed about the dimensions of the problem,'' Mr. Dicks said, adding, ''It was compartmentalized and disseminated over the years in dribs and drabs so that the full extent of the problem was not known until the Cox committee.''

Last fall, midway through the Cox panel's inquiry, Bill Richardson took over as Energy Secretary.

After being briefed by Mr. Trulock, Mr. Richardson quickly reinstated background checks on all foreign visitors, a move recommended 17 months earlier by the F.B.I. He also doubled the counterintelligence budget and placed more former bureau counterintelligence experts at the labs.