There are lots of dead cows in Texas on any given day.
Misquoting X-Files, "The Strain Is Out There".
There's a large Pakistani community in SE Texas. They are there for the climate AND chemical industry. There are enough of them they support a decent goat/sheep slaughtering business (as they do here in the DC area). You becha' Paks know about anthrax.
No, I wasn't assuming the letters were mailed abroad ~ that's clearly not the case. However, once the envelopes were filled with anthrax, the anthrax spores would begin filtering through the paper. That started immediately, not later. Because the guys doing the attack knew that this was going to happen they TAPED the envelopes. This involved wrapping the envelope end to end, on both sides, with a piece of clear packaging tape ~ the wide stuff!