The spores were able to infiltrate the paper and go right through the walls of the envelopes anyway.
My scenario would have Mr. Atta's friends dropping off 3 or 4 envelopes in local curbside collection boxes down there in the Boca Raton area.
Normal postal operations would then serve to bring those letters together in the same stack of "empty" trays. Here's how:
1. All the collection boxes were in a wealthy neighborhood - theirs! The USPS frequently places collection boxes in such neighborhoods even though there is rarely any mail dropped in them.
2. Each collection box has a large "flat tray" in the bottom. This is that larger postal tray made out of corrugated polypropylene that the general public likes so much - for gardening, in the garage, trash collection, etc.
3. When mail is dropped in the top of the collection box it drops into the tray in the bottom.
4. The carrier or collection route employee replaces the tray, taking the supposedly "full" tray and putting it in the back of the collection vehicle. That way no letter is missed - and you better believe, those rich old people with nothing better to do than watch the collection box know if the carrier has done the job correctly - and if not they will instantly call postal management and complain.
5. The carrier stacks the trays taken from the collection boxes up in her vehicle. At the end of the route, those trays are transferred to the collection platform.
Since these trays are virtually empty they will be mistaken for empty mail transportation equipment (MTE). Because Florida is a mail destination area and generates little of its own, lots and lots of postal equipment would pile up down there unless they shipped it back North. Those empty trays were stacked with other trays on a pallet, then wrapped in plastic sheet and shipped North to places that needed more empty trays - e.g. Philadelphia BMC which, in turn, supplies equipment to Trenton.
As the stack of trays was "worked", the letters were found and tossed into 010. That particular stack of trays was "worked" down over a period of days which is why I believe the trays were sent to a major mailer in the Trenton area and not directly to a postal facility. Later unexplainable contamination occured when those very same contaminated trays were sent around to other jobs.
Mail delivery has been chaotic since the 9/11 attack. Mail service was interrupted and delivery slowed down when the planes were grounded (mail continued to move on train).
Mail delivery is still affected. I had a Priority Mail envelope that took 2 weeks to arrive and it was mailed on October 25th.
Regarding the confusion as to whether the envelopes were sealed, it would be a good thing to know at any rate. I did hear (initially) that the envelopes (1, 2 or all 3?) were not sealed, rather the flap was tucked in. It could have been closed with tape.
Another small detail that was botched during reporting of these 9/11 incidents was that Atta played pinball the night before the attack. Later this "pinball machine" was identified as Golden tee, a golf simulation video game. The journalist who wrote the article that identified the machine still referred to it as a pinball machine. In the grand total, it means nothing. But it goes to show that journalists haven't got a clue as to what details are important and are making sloppy errors in reporting the "facts".