Not nearly enough data for the reader to determine this is home-grown:
The similarities between the pre-Sept. 11 and post-Twin Tower disaster letters has further fueled a theory at the FBI and Justice Department that the anthrax scare is the work of a twisted home-grown menace rather than a terrorist linked to state-sponsored action or Osama bin Laden.

Every plane used for the Sept 11 attack was from a U.S. airline, yet they were hijacked by aliens. There is no reason stated in the article for the same not to be true of the U.S. Postal System. The attack remains anonymous. The town under investigation could merely be a point chosen for the instigator to post the letters from, meaning he could be from just about anywhere, even arriving from Hawaii for the simple purpose of spreading terror.

As far as the average reader is concerned, this could still be an intricately planned operation from the Middle East,
Dan