This investigation had the “feel” of the Oklohoma City Bombing investigation - Quickly decide on a suspect and shape all the investigative evidence to fit the suspect.
What are the odds that the 9/11 terrorists and the Anthrax Mailer were totally and completely unrelated to each other. Someody with anthrax laying around just decided to up and start mailing letters days after the 9/11 attacks?
About the same as the probability that 9/11 was completely unrelated to the deadly airliner crash in NYC a couple of months later (i.e. 99.999% prabability).