Dr. Steven J. Hatfill, the former Fort Detrick biodefense researcher whose Frederick apartment was searched Tuesday by the FBI, commissioned a 1999 study that described a fictional terrorist attack in which an envelope containing weapons-grade anthrax is opened in an office.

The study, written by a veteran of the old U.S. bioweapons program, was submitted to Hatfill and a colleague at Science Applications International Corp., the McLean, Va., defense contractor where he then worked.

It discusses the danger of anthrax spores spreading through the air and the requirements for decontamination after various kinds of attacks. The author, William C. Patrick III, describes placing 2.5 grams of Bacillus globigii, an anthrax simulant, in a standard business envelope - slightly more than the estimated amount of anthrax in each of the letters that killed five people last fall.

The study, portions of which were read to The Sun by a person who has a copy, illustrates the central paradox of the FBI's nine-month quest for the anthrax mailer: The perpetrator could be a respected American scientist in the biodefense field, where he acquired the skills he then used to kill.

The study discussing the mail attack, for instance, was written as a scientific exercise to draw on Patrick's expertise and help improve defenses against bioterrorism. But the FBI must consider the possibility that such a document could have planted the seed for a terrorist plot.

The traits of a top-notch specialist in biodefense are the same as those of the likely perpetrator of the mail attacks: knowledge of anthrax and how it can be turned into a potent weapon; access to a lab where anthrax is stored; vaccination against anthrax; even very strong views about the threat of bioterrorism.

Hatfill, 48, is a colorful character with all those traits and more. He has said in interviews that his background naturally drew the FBI's attention. Attempts to reach him yesterday were unsuccessful, and the manager of his apartment complex told reporters he was traveling overseas. Hatfill has adamantly denied having anything to do with the anthrax mailings.

A physician and Ph.D. who completed Army Special Forces training, Hatfill is a pilot and has special training in aviation and submarine medicine. He spent 14 months as a doctor and researcher in Antarctica. More recently, he told his college alumni magazine that he has trained with the United Nations to become a bioweapons inspector in Iraq if the regime agrees.

Hatfill, raised in Mattoon, Ill., attended medical school in Zimbabwe, then Rhodesia, and has described witnessing in 1979-1980 the largest outbreak of human anthrax - an estimated 10,000 cases, most of them cutaneous. Experts still debate whether the Zimbabwe outbreak occurred naturally or was a tactic in the civil war then raging between the white government and black guerrillas.

In recent years, while working at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick as well as at the National Institutes of Health, Hatfill has spoken frequently on the bioterrorist threat, stressing how easy it would be for a terrorist to brew a deadly bioagent in his kitchen. While at SAIC, where he worked from 1999 until March, he helped create a mock bioterror laboratory for use in a training exercise in Guam for soldiers of the U.S. Special Operations Command.

Hatfill is a friend and protege of Patrick, 75, a bioweapons legend who has himself experienced the dual status of expert and possible suspect.

Recently, Patrick underwent a three-hour FBI polygraph examination. When he passed, the FBI invited him to join the inner circle of technical advisers to the investigation, Patrick said.

Another anthrax expert, Martin Hugh-Jones of Louisiana State University, said he, too, has been questioned repeatedly by the FBI, both as a scientist and as a possible perpetrator.

"Sometimes it's one and sometimes it's the other," he said. He doesn't like being grilled, but he accepts it. "I think they would have been derelict if they hadn't questioned me."

In the case of Hatfill, it is unclear why FBI agents waited at least six months after they first questioned him to conduct a thorough search of his home. One possibility: a briefing last week for Senate staffers by biologist Barbara Hatch Rosenberg.

Rosenberg, who heads a biological weapons working group at the Federation of American Scientists, has repeatedly criticized the bureau for failing to aggressively pursue a "likely suspect" whom she has not named but who closely resembles Hatfill. Her Senate briefing was attended by Van Harp, who heads the anthrax investigation as assistant FBI director in charge of the Washington field office, and three other FBI agents.

FBI officials, speaking on background, say that Hatfill is only one of many scientists who have come under scrutiny, that he agreed to the search and that they found nothing incriminating, though tests for anthrax spores are not complete.

Yet neighbors and television viewers will not soon forget the daylong spectacle of FBI agents, some in protective gear, carrying equipment in and out of Hatfill's apartment just outside the gates to Fort Detrick.

In March, in a telephone message to The Sun, Hatfill complained that his very dedication to the cause of biological defense had brought him under suspicion. He said he had just been fired from his job at SAIC, the defense contractor, and blamed news media inquiries.