After years of forensic examination, the FBI concluded that the anthrax used in a series of postal attacks almost seven years ago had such a unique scientific signature it could only have come from a single flask controlled by government scientist Bruce Ivins, according to newly unsealed government documents. The flask, known as RMR-1029, was a persuasive piece of evidence convincing prosecutors that he was the principal and, officials say, the sole perpetrator of the attacks.

Ivins, who worked at a government lab at Fort Detrick, Md., until he committed suicide last week, is depicted in the documents as secretive and obsessive and suffering from long-term mental illness. Even so, much of the evidence laid out by the FBI in thousands of pages appears to be circumstantial.

For instance, e-mails written by Ivins after the attacks of September 11, 2001, appear to match the childish screeds in the anthrax-laded letters. And around the time that the anthrax letters were mailed from Princeton, N.J., Ivins began working late at night and, investigators believe, was taking long late-night drives.

According to the documents, which the chief federal district judge in Washington, D.C., ordered to be unsealed today, the FBI had been dealing with Ivins, a senior biological weapons researcher, since shortly after the first anthrax postal attack was reported. Initially Ivins, an expert in anthrax vaccines, was regarded by investigators as a potential collaborator and source of information. But last year, investigators began to focus on him as a suspect. His lawyer has said Ivins had no part in the attacks.

E-mails written by Ivins to a friend between April 2000 and December 2001 showed increasing mental stress. According to an FBI affidavit, in an e-mail sent Sept. 26, 2001—a couple of weeks after the first anthrax letters were mailed—Ivins wrote to his friend that Osama bin Laden "has just decreed death to all Jews and all Americans." FBI investigators noted the language was similar to what was used in the anthrax letters postmarked two weeks later, warning "DEATH TO AMERICA" and "DEATH TO ISRAEL."

According to the FBI reports, Ivins's normal working hours were 7:30 a.m. to 4:45 p.m., spent in a laboratory especially equipped for bio-weapons research known as Suite B3. Periodically, according to the FBI, he would return in the evenings, presumably to check on experiments. Beginning in mid-August 2001, however, the FBI noted what it called a "spike" in his evening access to the B3 suite. Immediately before letters containing anthrax were sent to the NBC News anchor Tom Brokaw and the New York Post around Sept. 18, 2001 (the postmark date), the FBI established that Ivins had spent unusually long night hours in his lab.