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. The former agent, Richard L. Lambert, who spent 24 years at the F.B.I., says he believes it is possible that Dr. Ivins was the anthrax...