Years ago, he had visited Kappa Kappa Gamma sorority houses at universities in Maryland, Virginia and West Virginia, an obsession growing out of a romance with a sorority sister in his own college days at the University of Cincinnati  although someone who knew him well said the last such visit was in 1981.

What is more relevant, agents focused new attention on a 2002 Army investigation of a spill of anthrax the same year outside the secure laboratory that Dr. Ivins worked in, and his puzzling behavior in trying to clean the area with bleach while failing to report the contamination. They studied his anthrax vaccine patents and considered whether the promise of royalties after a bioterrorism scare might have been a motive. They noted that he had a lyophilizer, which could be used to dry wet anthrax into powder, a form not ordinarily used at Fort Detrick.

They had even intensively questioned his adopted children, Andrew and Amanda, now both 24, with the authorities telling his son that he might be able to collect the $2.5 million reward for solving the case and buy a sports car, and showing his daughter gruesome photographs of victims of the anthrax letters and telling her, “Your father did this,” according to the account Dr. Ivins gave a close friend.

As the investigation wore on, some colleagues thought the F.B.I.’s methods were increasingly coercive, as the agency tried to turn Army scientists against one another and reinterviewed family members.

One former colleague, Dr. W. Russell Byrne, said the agents pressed Dr. Ivins’s daughter repeatedly to acknowledge that her father was involved in the attacks.

“It was not an interview,” Dr. Byrne said. “It was a frank attempt at intimidation.”

Dr. Byrne said he believed Dr. Ivins was singled out partly because of his personal weaknesses. “They figured he was the weakest link,” Dr. Byrne said. “If they had real evidence on him, why did they not just arrest him?”

Another former co-worker, Dr. Kenneth W. Hedlund, who collaborated on anthrax research with Dr. Ivins in the 1980s, had a similar theory.