If all else fails, they can put Gonzalez in prison for contempt of congress. Don't drop the soap, Alberto
Such conspiracy theories couldn't get off the ground without the ill-advised clouds of silence from those (supposedly) in charge. And some conspiracy theories turn out to be true. A more sober appraisal of the anthrax scare.
The latest article I've seen on this said that the anthrax in the letters was very pure, but did not have the additives to increase it's virility that you would see in weaponized agent. Basically, it was labratory grade but not weaponized and therefore the supposition that it was someone with expierience in the bio-warfare industry seems less likley. Of course, the counter-theory is that it was someone with industry expierience who intentionally made a less lethal product for the purpose you outlined.
It seems obvious that if someone wanted a major scare without a major death toll they'd go for the watered-down version. The letters also made it instantly obvious that the contents were highly dangerous - to cause panic rather than to cause an epidemic. Definitely a home-grown plot.