Re: Notice anything unusual about the letters?
I still think it comes down to the postmarks (although they could be faked).
If in fact they were mailed prior to 9-11 then it is highly (make that extreemly likely) that they are part of the 9-11 attacks.
I couldn't really read the postmarks, but it looks like two of them were dated on 9-11 which could still indicate a domestic terrorist group. So you may be right.
I have a suspicion that there is a good deal of evidence that the FBI is holding back from release. Time will tell. I hope they find and fry those responsible.