It's, also, noteworthy that there have been simultaneous anthrax attacks in other countries that most Americans can't even find on a map. A newspaper in Pakistan received an anthrax letter, anthrax attacks in India, etc. I think this is international terrorism in response to a Muslim call to jihad and related to the anthrax attacks in the U.S.
I'm not familiar with the incident(s) in India, but the ones in Pakistan turned out to be a hoax. They had tested positive with the very inaccurate field test, but the comprehensive tests proved positively that they were not anthrax.
We can't rule out anything yet. My personal preference would be that this could somehow be tied back to Iraq so that we could raise worldwide indignation for our upcoming action there.
But the evidence pointing there is very weak so far. Perhaps more will surface.