I wouldn't consider the Army of God to be mainstream or conservative, but way out on the fringe.
If this letter tests positive for anthrax after lab testing (and the test does give false positives--which is a whole lot better than false negatives), the Army of God would become radioactive. Neal Horsely and his buddies will be in deep deep doo.
For that matter, anyone can write The Army of God. Easy to finger these people that's for sure because they have unpopular opinions.
The Afghans do have a beef - the UN Population Fund using U.S. tax dollars is sending "reproductive health service products" which includes vaccum aspirators (also sent to the Bosnian women) to kill their unborn children. I consider that a serious motivation for the FBI to explore .
Before we start singling out, people ,we don't like, we better have the facts.