Apparent flimsiness of anthrax case

Despite the FBI offering to release their evidence against Dr. Ivins, their case against him in the anthrax case still seems to be rather flimsy. At least to me, it seems that their entire case is based on him being eccentric and that he had access to the strain of anthrax used in the attacks.

He may or may not have been responsible, but I find it hard to conclude that Dr. Ivins was responsible. I know a lot of people who are as comparably eccentric (I'd say that would include me, and I can't conceive myself of hurting someone else). And with a hundred people also having had access to the same strain, the FBI hardly has a slam-dunk case. Even though there are plenty of cases where a hypothesis is strongly supported only in the context of the accumulated evidence, this doesn't look like one of them, despite claims to the contrary by the FBI.

Is there some public information that I'm missing? Maybe I missed a press release by the FBI that presents a strong case. Or is the FBI desperate to close the case only for political reasons? At least the science behind their claim that the strain of anthrax used in the attacks came from Fort Detrick sounds fascinating, although I don't have the competence to judge how strong it is.