The FBI searched the apartment yesterday of a biological weapons scientist in Frederick as part of the continuing investigation into the mailing of anthrax-laced letters that killed five people last fall.

An FBI car and Ryder rental truck were parked yesterday evening outside Detrick Plaza apartments, and agents were carrying out large trash bags filled with unknown materials collected inside, a witness said. The low-rise apartments are just outside the main gate to Fort Detrick.

The scientist agreed to the search in the hope that it would remove his name from the list of possible suspects in the investigation, one law enforcement official said.

Dr. Steven J. Hatfill, 48, has not been charged or identified by the FBI as a suspect. He worked at the U.S. Army Medical Research Institute of Infectious Diseases, the top military bioterrorism research facility, for about two years in the late 1990s.

Hatfill, who has an M.D. and a Ph.D. in molecular cell biology, has conducted scientific research and training in the field of bioterrorism, including training for emergency personnel and U.S. special forces troops.

Like other researchers in the field, he has been vaccinated against anthrax, has had access to labs where it is stored and has some knowledge of its use as a weapon, according to former colleagues. Those factors brought his name to the attention of the FBI several months ago.

He could not be reached for comment last night.

He was interviewed by FBI agents and given a polygraph test early this year, which he passed, he said in an interview with The Sun in February.

Reached at his job with a government contractor, Hatfill said then that he considered the questioning to be part of a routine effort to eliminate people with the knowledge to mount an anthrax attack.

In the February interview, he explicitly denied having anything to do with the attacks.

"I think they had a profile," the scientist said. "They had a bunch of people on the list. They have to rule people out. ... I certainly didn't appreciate getting called in. No one likes that. I'm one of the good guys."

Hatfill has worked in recent years for the McLean, Va., office of Science Applications International Corp., a large defense contractor. But the government lifted his security clearance in July 2001, according to SAIC officials.

After waiting to see whether the clearance would be restored, SAIC dismissed Hatfill in March, the officials said.

In 1997, Hatfill told a Washington Times columnist it would not be hard to mount a biological attack. "Dr. Hatfill, who is familiar with such things, showed me how to culture bacteria with supplies that can be bought at Safeway," wrote columnist Fred Reed.

The next year Hatfill, then at the National Institutes of Health, was photographed for Insight magazine demonstrating "how a determined terrorist could cook up a batch of plague in his or her own kitchen, using common household ingredients and protective equipment from the supermarket."

Last night, another law enforcement source suggested that the search of Hatfill's home was only one among many that have been or will be conducted.

The FBI has been administering voluntary polygraph tests to workers at Fort Detrick and at Dugway Proving Ground in Utah, where the Army conducts tests of equipment designed to detect biological weapons and decontamination methods.

Dugway is the only U.S. facility known to have manufactured small quantities of dry, weapons-style anthrax powder in recent years for use in tests.

At Detrick, researchers have used only wet anthrax mixtures.