This is very telling.
The FBI could have said "we will prove to Congress, the scientific community and the public that only Ivins could have done it".
But they didn't.
Instead the FBI is trying to discredit the many top anthrax scientists who question the government's case against Ivins by using the "grassy knoll" conspiracy-theory smear.
If they had a case against Ivins, they would have presented it, instead of resorting to Bill O'Reilly style smear tactics.