If you'd been following the mailflow aspects like I have you would have seen that the FBI's scenarios all assume a continual stream of mailings, a letter here, a letter there, and NOTHING before 9/11.
We should be heartened however since they actually picked these guys up, and others, whose involvement could only have been IF the preparation of the letters if not their mailing occurred before the plane crashes. I am still holding out for at least one letter being entered at the main post office at Boca Raton, right in the only letter drop ever to have been found to have even 1 anthrax spore! The others would have gone into local collection boxes.