WASHINGTON — More than eight years after anthrax-laced letters killed five people and terrorized the country, the F.B.I. on Friday closed its investigation, adding eerie new details to its case that the 2001 attacks were carried out by Bruce E. 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 F.B.I. history, laid out the evidence against Dr. 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...” Dr. 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 distress, Dr. 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 F.B.I.’s theory that Dr. 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.