As you point out, the 4th grade class reference wouldn't be meaningful to an alien in this country.
The use of prestamped envelopes and unlicked flap indicate someone who's watched all the Discovery or TLC detective series where someone is nailed by his DNA!
The "09-11-01" at the top of the letters is trying too hard to pin it on OBL and company. I don't Osama's boys would feel the necessity of identifying themselves if they were the anthrax perps.
Finally, the misspelling of penicillin as "penacilin" in one of the notes (Brokaw, I think) indicates to me that this is a native english speaker. If I were going to write a threatening letter in some language I wasn't very fluent in, I'd pull out the dictionary to look up the technical terms. It reminds me of Internet-speak, where few authors bother to get the spelling right but just type it out the way it sounds.
And what's with the "09" for September? I've seen a lot of references on FR and elsewhere to the events of 9/11/2001 but I don't recall anyone writing it as "09-11-01".
The various eccentricities about these notes are eventually going to nail this guy.
The 'penacilin' spelling beards the cat.
Otah , an English speaking individual would also seek the correct spelling of a word if in a foreign nation...
Zip codes in a foreign country usually precede the city or town...
Great brainstorming......