[NixNatAVanG InDaBurgh]

It should have occurred to someone that it wasn't a letter which started the anthrax rolling. It should also have occurred to someone that other things pass through the mail, like shipping boxes via FedEx or other containers. It might also have occurred to someone, that ANTHRAX isn't picked up in an airport screening process and could easily have been hand carried from one place to another in a plastic bag or sealed medicine bottle made of either glass or plastic. It could have been carried in a locket or pocket watch, a sealed joint, cocaine or other powder in gelcaps, an inhaler.
The letters on the documents say ATT ATTA, not simply ATTA.
The origination of the anthrax was NOT in Trenton, and the letters did not originate there either. They were delivered there and sent from there, but there is no lab to be found, no ghostly inhalation or cutaneous anthrax delivered through walls from one location to another. I am surprised that this analysis hasn't considered diplomatic courriers from places which could easily have brought this crap directly to any embassy which is connected to any *state* which is known for research in high tech bio fields...Saudi Arabia being very high on the list.
But, hey. What do I know?

[Alamo-Girl]

Hmmm ... thank you so much for all your observations!

The consensus to date is that the anthrax was not made by the perp but was provided to him.

[Nogbad]

Any country quickly could produce aerosole anthrax derived from the Ames strain once the recipe was known.

According to the Wall Street Journal at most 8 labs have the virulent strain of anthrax that the terrorists used,
and, reading between the lines, they all have been checked and cleared.
I'd say they know where the anthrax came from, they just aren't telling.