On Sept. 14, the FBI says, he spent from 8:54 p.m. until 12:22 a.m. at the lab; the next day, a Saturday, he spent from 8:05 p.m. until 11:59 p.m. in the lab, and the following day, a Sunday, he spent from 6:38 p.m. to 9:52 p.m. at the lab. The FBI documents say that Ivins established a similar pattern of late-night visits to his lab during the weekdays and weekend right before Oct. 9, 2001, the day that anthrax letters to Sens. Tom Daschle and Patrick Leahy were postmarked.

The FBI reports say that during the course of the investigation, Ivins was asked by the FBI to provide various anthrax samples for use in the inquiry. Samples he submitted to the bureau in 2002 turned out to be "unusable" due to Ivins's alleged "failure to follow" rules regarding the samples. The bureau later confronted Ivins about his actions, but he denied there was anything irregular in his behavior. Investigators now regard Ivins actions as evidence that he was trying to throw them off his trail.

Much of the other evidence against Ivins is even more circumstantial, though at a Justice Department press conference today, government officials said they believe that as a whole it demonstrates Ivins's guilt beyond a reasonable doubt.

According to the government's dossier, scientific analysis indicates that the anthrax used in the attacks came from a flask whose "sole custodian" had been Ivins since the anthrax strain was first grown in 1997. At the press conference, officials said that Ivins had total control over who had access to the contents of the flask. They also said that they had investigated everyone else who might have had access to the flask's contents and ruled them out as suspects.

Investigators noted pointedly that Ivins also had access to a lyophillizer—or freeze-drying machine—which the attacker used to convert the normally wet pathogen into a dry powdered form that was likely to be more deadly and effective in postal attacks.

What Ivins's motive might have been remains unclear. The government account suggests that the dead man's bizarre obsessions and serious long-term mental illness at least partly explain his actions. The unsealed FBI reports confirm that investigators established that Ivins had been fixated, perhaps since his own university days, with Kappa Kappa Gamma, a college sorority whose Princeton chapter was located 60 feet away from a mail box where some of the anthrax attack letters were posted. At today's press conference, however, investigators acknowledged that they had no piece of specific evidence—such as a gasoline receipt—which establishes conclusively Ivins's presence near the Princeton mailbox around the time the anthrax letters were mailed.