Two reasons for that knowledge -
1. They observed that the first victim in Florida probably had to be infected before 9/11, and that he had a fairly close relationship at one degree through his wife who had been the rental agent for an apartment where some of the hijackers lived (until they died on 9/11).
2. They read my posts. Unfortunately I do not have access to any investigative materials except those published in the media or announced on TV, but I know enough about the operating environment in which the attack took place to point out the nonsense and separate it from the good stuff, and there was good stuff in there - unfortunately never from the FBI, but certainly from the Inspection Service.
At this point I don't have enough information from this article to say "case closed", but we finally have someone at the federal level understanding that the anthrax attack took place prior to 9/11, and the supposed deaths would coincide with the attacks on the buildings.