I made that same mistake the other day -- the media kept talking about letters of September 18th, as if that was when they were posted. It was just this week (yesterday, I think) that they actually showed the envelopes with the postmark of September 11.
The first thing that comes to mind is that they were posted the night of Sept. 10 or before they headed to the airport Sept. 11. Or were posted by cohorts left behind on the ground who may still be among us or who left on the flights later that week. And I would think that they originated from the group that flew out of Newark because the others started their flights from Maine and DC.
I still want to know what happened to the 12 Middle Eastern men, posing as pilots, who were reported to be riding in the cockpits of planes grounded after the attacks of Sept. 11 started. Have they been rounded up? Or are they still among us?