They were steered in this direction primarily by an "expert" named Barbara Hatch Rosenberg, with some help from New York Times editorialist Nicholas Kristof.
I don't think that either Occam or Andre Gide were forensic pathologists.
It's true the particular strain originated in the United States. However, this really doesn't mean anything when you consider that the same strain has also been in the hands of the British and the Russians for quite some time now, and that's simply to the best of our knowledge. God only knows how many people could have possibly stolen a tiny sample of the strain from any one of those places. Security was a complete joke during the Clinton years, and our government today might as well be the U.N.
If you manage to get one of my hairs or a drop of my blood, you have my DNA. With enough scientific expertise, you can replicate it and plant it to make it look like I was somewhere, even if I was never there, so it's not as though forensic pathology is completely foolproof.