The way the letters were folded suggests a SYRINGE type insertion of the anthrax into the letters. But this could have been done in the woods, were any residue would quickly disperse.
It's still the refining process that gets me because it likely took many attempts to refine it to their specs. An Arab student who studied here briefly and who got the basics still comes to mind....Just like the pilots using our training schools. Saddam has the Chemical forte and Usama has the Structural forte. Patience....We have Ms. Anthrax. Saddam himself knew @#$% about Anthrax.
I wonder if there were any holes in the letters or envelopes which is why they sealed the edges which are not necessarily the weakest part of an envelope. (There's just a lot that they're not telling us.)
One report suggests that there were slits in the envelopes through which a hypodermic needle could have been inserted.