Mitch,there WERE a few other things that match the September and October dates: On Sept.17 (as I recall) Pres. Bush publicly identified Osama as the "author" of 9/11;and on October 18 (again,as I recall)we began bombardment of AQ and Taliban in Afghanistan.
In Princeton, NJ-not far from where the second batch of letters was mailed,there were 2 large student demonstrations going on: one pro-war;the other anti-war.
Both demonstrations "collapsed" when word of the Afghanistan response was received.(Both factions had been on tenterhooks because the media had been speculating on the date for quite a while.)
I can picture someone at one of the demonstrations (the "Anti",seems more likely to me)hurrying away to a storage spot,retrieving a carefully wrapped package,and walking it to the mailbox,in direct reaction to the news.
I don't believe it was something as casual as that. The weaponized anthrax could have caused very large numbers of casualties, if it had been delivered differently. The use of what is clearly a WMD would have been planned out in advance.
Any theory must explain the motivation of the mailers -- what is the motive that would cause them, on the one hand, to use the anthrax but, on the other hand, to use it in a way that would minimize its military effectiveness?
And what is the motivation for mailing two very liberal Senators?