Notice anything unusual about the letters?
http://www.fbi.gov/pressrel/pressrel01/102301.htm
It's the dates. They read 09-11-01. I would have figured that an international terrorist would have dated it 11-09-01 (the date format used in most places outside the USA?). Does this mean it was a domestic terrorist? Or an international terrorist merely conforming, perhaps even without thinking about it, to the local standard? Not sure.