When Bruce E. Ivins, an Army microbiologist, took a fatal overdose of Tylenol in 2008, the government declared that he had been responsible for the anthrax letter attacks of 2001, which killed five people and set off a nationwide panic, and closed the case.

Now, a former senior F.B.I. agent who ran the anthrax investigation for four years says that the bureau gathered “a staggering amount of exculpatory evidence” regarding Dr. Ivins that remains secret.