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 two years in the late 1990s.

He 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 attributes 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.

"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."

Last night, another law enforcement source suggested that the search 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.

The FBI has come under increasing pressure to solve the anthrax case from scientists who accuse it of bungling and delay. Barbara Hatch Rosenberg, a biologist at the State University of New York, recently circulated among bioweapons specialists an analysis that describes the former Fort Detrick scientist as a leading suspect but does not name him.

In addition, the bureau has faced pointed inquiries from Congress and outside experts who question its focus on domestic laboratories and wonder whether Iraq or the al-Qaida terrorist network might be responsible for the mailings.