It's been ten years since the attacks and there aree still many unanswered questions. The FBI "closed" the case three years ago only to have their scientifc evidence called into question. Even if they are right that the attacks were carried out by Bruce Ivins (I have my doubts), their investigation was a mess.
For instance, we can add Perry Mikesell to the list of innocent men (Brian Kelly, Richard Jewell, Steven Hatfill) whose lives were ruined by Bureau:
One early suspect was microbiologist Perry Mikesell, who had worked in 1999 at Battelle. Under the pressure of the FBI's scrutiny, he drank heavily and had a fatal heart attack in October 2002. According to family members, he had drunk himself to death.
Ivins emerged as the final suspect by a process of elemination.
It now turns out that the National Academy of Sciences panel found that "it is not possible to reach a definitive conclusion about the origins of the B. anthracis in the mailings based on the available scientific evidence alone." As Epstein notes: "without a scientific basis for tracing the killer anthrax to Ivins's lab, the FBI's case against him was reduced to inferences from his behavior."
Among the anthrax samples collected from different labs, only one matched the "fingerprints" of the killer anthrax in the letters. It was from flask RMR-1029 in Ivins's lab. This anthrax had been created in 1997 at the Dugway Proving Ground and sent to Ivins for tests. The FBI considered all the other scientists whom Ivins had given access to RMR-1029, then eliminated them as suspects for one of three reasons: They had not worked solo in their labs and thus lacked the privacy needed to process the wet spores into dry powder; they lacked the skills to do the job; or they were too far away to mail the letters in Princeton. Through this process of elimination, the FBI arrived at Ivins, who worked alone in his lab, had the skills and could have driven the nine-hour round trip from Frederick, Md., to Princeton and back.
Inferences from behavior is notoriously unreliable in crime solving. (See here and here).