http://georgewashington2.blogspot.com/2008/08/top-anthrax-expert-maybe-theres-two.html

William Patrick is the leading U.S. expert on weaponizing anthrax, and has multiple patents in anthrax weaponization.

Patrick made an interesting point in a November 2001 article in USA Today:

Further complicating things, the spores contained in the first attack, on a Florida media company, have been described by investigators as a clumpy powder, which would make a poor aerosol. Only two weeks later, the finely prepared Daschle-letter spores appeared. Those spores were ground so fine that they apparently drifted across offices and contaminated other letters in the mail. The same strain of anthrax was used in all the attacks, suggesting a common source. The bioterrorists "must have had a hell of a short learning curve," says Patrick, who headed U.S. bioweapons work until the program's halt in 1969. "Or maybe there's two groups."

In other words, either a single killer or group of killers went from a crude anthrax preparation to an extremely advanced weaponized form of anthrax in 2 weeks, or there were two groups of killers.

The anthrax sent in the first letters was coarse, brown, "sandlike", and clumpy. The anthrax sent to Daschle and Leahy was white and very fine. As the Washington Post noted, it was:

1.5 to 3 microns in size and processed to a grade of 1 trillion spores per gram -- 50 times finer than anything produced by the now-defunct U.S. bioweapons program and 10 times finer than the finest known grade of Soviet anthrax spores.

In addition, the anthrax sent to Daschle and Leahy was so advanced that no one at Fort Detrick had ever seen anything like it before, and scientists concluded that it used nanoparticles of polymerized glass to ensure dispersion.

In other words, if a single killer sent out all of the anthrax letters, he started out with a very crude, raw anthrax sample, and ended up with the most highly-advanced weaponized anthrax in the world, one which many leading experts say would have required expensive high-tech equipment, a team of people, and many months to prepare.

Indeed, why would someone send a crude anthrax sample if they were going to end up sending a highly-advanced preparation soon after? Why not just wait until the weaponization was complete, and then send both? That doesn't make sense.

Two (or more) separate groups of killers is much more likely.