I haven't got a clue as to who did this--what were the results of the police/postal inspector's investigation? If they don't know--and I haven't seen anything in print about any arrests in this case--I surely don't know. Do you? But anyone who sends a threat through the mail, or mails faux or real anthrax should be charged and prosecuted to the fullest extent of the law.
Just with the current situation, unless there's definitive evidence that says this person or this group is a suspect, it could have come from anyone. As the cops say, "everyone's a suspect," and that's how they investigate, or should investigate.
BTW, Horsely did the same bonehead stunt someone did at the Green Bay Police Department yesterday: rather than putting down the envelope and calling the police, he took the envelope with the suspicious powder to the police department. The GBPD ended up quarantining their lobby until the hazmat team could do their magic and clean up & clear the place.