It is impossible for the FBI to know where the hijackers were minute by minute for the months they were here. All they can do is check out known places. It's not also out of the realm of possibility that they could've passed on the anthrax to an associate from another al Qaeda cell or an al Qaeda operative with instructions to mail out the anthrax post-9/11 to cause more chaos and terror.
We are looking through a peephole at the backsides of FBI agents looking through another peephole.
1% of 1% of the facts is not much certain knowledge.