I don't think it's some terror group, and I do believe the anthrax came from US gov't labs. The actual FBI narrative, and how the media had been fed the story by anonymous sources for uncritical regurgitation, is the matter of interest beyond the whodunit. The official narrative is full of holes, and we don't know the what the truth of the situation is. There are some who think the Anthrax scare was a false flag operation/inside job meant to scare the senators in question into passing the Patriot Act quickly. The Anthrax attack was also initially linked (thanks to more anonymous sources who should really be outed now) to Iraq in an attempt to beat the drums of war.
I don't have much of a theory myself, but what the FBI's been pushing out is not reason enough to say "he did it, he did it alone, case closed." There are a lot of weird aspects to this whole situation. Even if the official line is to be believed, it displays obvious incompetence on both the part of the FBI and Ft. Detrick, but there's no OMG scandal. Either way the FBI apparently drove this guy to suicide, which doesn't seem like a very good way to solve a case.
He's not the first guy they've tried to pin this on (see
http://en.wikipedia.org/wiki/Steven_Hatfill ). They want this whole thing to be over and done with... it's a loose end to tie up before January 20th.
It looks like around here only Eric P and I find it particularly interesting... maybe it's just a side effect of living inside the beltway.