Bruce Ivins

WASHINGTON — More than eight years after anthrax-laced letters killed five people and terrorized the country, the FBI on Friday closed its investigation, adding eerie new details to its case that the 2001 attacks had been carried out by Bruce Ivins, an Army biodefense expert who killed himself in 2008.

A 92-page report, which concludes what by many measures is the largest investigation in FBI history, laid out the evidence against Ivins, including his equivocal answers when asked by a friend in a recorded conversation about whether he was the anthrax mailer.

“If I found out I was involved in some way…,” Ivins said, not finishing the sentence. “I do not have any recollection of ever doing anything like that,” he said, adding, “I can tell you, I am not a killer at heart.”

But in a 2008 e-mail message to a former colleague, one of many that reflected his deep mental distress, Ivins wrote, “I can hurt, kill, and terrorize.” He added: “Go down low, low, low as you can go, then dig forever, and you’ll find me, my psyche.”

The report disclosed for the first time the FBI’s theory that Ivins embedded in the notes mailed with the anthrax a complex coded message, based on DNA biochemistry, alluding to two female former colleagues with whom he was obsessed.

The report described how an FBI surveillance agent watched in 2007 as Ivins threw out an article and a book, Douglas Hofstadter’s “Godel, Escher, Bach: An Eternal Golden Braid,” that could betray his interest in codes, coming out of his house in Frederick, Md., at 1 a.m. in long underwear to make certain the garbage truck had taken his trash.

Dropped into a mailbox in downtown Princeton, N.J., the anthrax letters were addressed to news media organizations and two U.S. senators and contained notes with radical Islamist rhetoric that appeared to link them to the Sept. 11 attacks, which occurred a week before the first of the two mailings.

In the jittery wake of 9/11, they set off a nationwide panic over random discoveries of white powder that people feared might be more anthrax. The real anthrax — a few teaspoons of very fine powder — infected at least 22 people, including several postal workers, and killed five.

Congressional offices and the Supreme Court were evacuated as a result of anthrax contamination, and the Postal Service spent hundreds of millions of dollars to clean up mail-processing centers. The federal government increased spending on biodefense, with a total of nearly $60 billion since 2001, and rejuvenated the faltering military anthrax vaccine program on which Ivins had worked for many years.