I've located images of 4 different versions of the treat letter. I also do a brief analysis of the handwriting.
While it's clear that the letters were NOT written by the same person who wrote the anthrax letters, it still seems that the motivation was similar: The culprit wants people to think that al Qaeda is about to launch another attack.
Hey, maybe Secretary of Homeland Security Michael Chertoff is the culprit! I just heard him not very many days ago say that he had a gut feeling Al Qaeda was preparing to launch another attack against us.