The former Army researcher, Dr. Steven J. Hatfill, allowed agents to search his Maryland apartment and Florida storage facility this week to clear himself from suspicion.
Investigators believe anyone skillful enough to send the anthrax letters without becoming sick must have had extensive experience. The FBI drew the group of 20 or 30 from 200 scientists whom it wants to look at further, the official said Thursday on condition of anonymity.
The bureau has searched about 25 homes or apartments after getting permission from the person interviewed, a federal law enforcement official said.
Law enforcement officials said on Thursday that Hatfill, 48, is not a suspect and no evidence links him to the letters.
Hatfill denied involvement in the anthrax mailings and complained to The (Baltimore) Sun in a March telephone message that he was fired from his job because of media inquiries.
"I've been in this field for a number of years, working until 3 o'clock in the morning, trying to counter this type of weapon of mass destruction, and, sir, my career is over at this time," Hatfill said.
Hatfill and another scientist, Joseph Soukup, commissioned a study of a hypothetical anthrax attack in February 1999 as employees of defense contractor Science Applications International Corp., said Ben Haddad, spokesman for the San Diego-based company.
The study, written by bioterrorism expert William C. Patrick III, describes placing 2.5 grams of Bacillus globigii, a simulated form of anthrax, in a standard business envelope, The Sun reported.
Hatfill did not answer the door at his apartment Thursday in a complex of brick, three-story garden-style apartments near Fort Detrick, Md.
He worked for two years at Fort Detrick in the virology division of the U.S. Army Medical Institute of Infectious Disease on a fellowship from the National Research Council, said Chuck Dasey, a spokesman for the base.
Hatfill stopped working there in September 1999 and was employed by Science Applications International until March 4.
Another U.S. law enforcement official, also speaking on condition of anonymity, said Hatfill's Defense Department security clearance expired and never was renewed. Such clearances must be renewed every five years.
He is a 1983 graduate of the University of Zimbabwe Medical School, according to the university's Web site. Investigators confirmed Hatfill graduated from the school.
ABC News reported this week that the FBI was interested in Hatfill partly because he lived, while in Zimbabwe, near a Greendale elementary school. "Greendale School" in Franklin Park, N.J., was printed in large block letters as the false return address on the anthrax-laden envelopes sent to Sens. Tom Daschle and Patrick Leahy last fall.
One investigator cautioned the FBI has been unable to place Hatfill near Trenton, N.J., during the time the anthrax letters were mailed. Officials believe the letters were mailed from the Trenton area.
Hatfill was first interviewed by the FBI in December, the investigator said.
Although Hatfill probably had access to anthrax in labs shared with bacteriology researchers, his primary duties didn't involve working with it, Dasey said.
Five people died in the anthrax attacks that began in late September. One of the dead was Robert Stevens, a photo editor for a tabloid newspaper headquartered in Boca Raton, 230 miles southeast of Ocala, where Hatfill's storage facility is located.