If you look at the other attacks you'll see big explosives and lots of possible casualties.
The anthrax scare has fewer than a handful of deaths.
Realistically, they'd get more casualties with a rifle or grenade during rush hour. More effective, too.
I don't know who might be doing it, but it doesn't look like ObL's work (or the work of his associates).
The world is a lot more complex than our government would like us to believe.