If Atta was involved, he was probably experimenting with other chemical combos. He would have lacked the sophistication to produce anthrax of that grade. He or his contacts may have been in collusion with the American scientist, or the scientist could have been working independently.
I don't have a strong sense about any of it other than what I posted on the original anthrax threads months ago.
It is possible that the anthrax cooker was paid by Atta and his buddies, and delivered the stuff to them, then Atta and his friends put it in letters, sent it off to be cycled through the mail in NJ and DC. Maybe that lady who died in NY, who had no known connections, mailed a letter for a friend of a friend and ended up with the disease herself.
The folks up here surmised that Ottille Lundgren, woman who died in CT, probably contracted it because of her habit of ripping junk mail into pieces before throwing it in the trash. Some of her mail had come through one of the affected sites. Even if the amount of anthrax was very small, she was older and had less of a chance of fighting the effects of the disease on her lungs.