First of all, why do you assume that the men who hijacked the planes were the only ones in America at the time or the only al-Qaeda in the plot? Isn't it likely that there were others supporting the plot? That's what the government seemed to think since they had a nation wide manhunt afterwords and did in fact find some evidence that there were helpers.
Second, why do you assume that the anthrax letter that killed the first person in Florida ... the one that worked a few miles from where the hijackers were staying ... was mailed after 9/11?
When did Stevens contract the anthrax? Some seem to think it happened on September 19th from a letter (the Lopez letter) sent to AMI. But when was the letter mailed and received? MSNBC quoted Newsweek (
http://www.msnbc.msn.com/id/3067576/site/newsweek/) saying that the Lopez letter arrived a week
before 9/11.
This site,
http://www.postalmag.com/editorial14.htm, dedicated to postal employees, also says that the Lopez letter arrived the 4th. Here's another site that says the 4th:
http://www.anthraxinvestigation.com/anthraxreport.htm.
Newsweek said they spoke to someone (unnamed) at AMI. And I've yet to find a source claiming the letter arrived on the 19th. Perhaps this might resolve the question:
http://anthrax2001.blogspot.com/ "The media reports on the AMI letters confirm what the CDC reports in many important details The media reported that Bob Stevens was indeed seen by his colleagues holding a letter close to his face on September 19, 2001. It was pointed out by Phil Brennan writing for Newsmax that this letter that Stevens was seen reading had actually arrived at AMI on September 4, 2001." Phil Brennan said Stevens held it up to his face and then put it down on the keyboard (where traces of anthrax were found). So maybe Steven's contracted the anthrax on September 19th, however the letter arrived at AMI before 9/11. And if it arrived before 9/11 to coincidentally infect someone working within a few miles of where the hijackers stayed, that sort of rules out a domestic terrorist, doesn't it?
Or maybe the source wasn't that letter. We really don't know. But we do know Stevens started showing symptoms some time before October 2nd, when he was hospitalized. This somewhat authoritative report (
http://www.fpd.umn.edu/files/GlobalChron.pdf link no longer working (and I don't care to figure out why at this time) said the onset of symptoms was around September 28th. Inhalation anthrax has an uncertain incubation time (from less than a week or two to as much as 2 months). The median time is reported to be 10 days according to one study. The CDC says its generally less than 2 weeks but "due to spore dormancy and slow clearance from the lungs, the incubation period for inhalational anthrax may be prolonged." And according to CNN, Florida Health Secretary Dr. John Agwunobi advised anyone who spent more than an hour in the AMI building
since August 1st to report for testing. Just to give you an idea of how uncertain officials might really be about the timeline.
The bottom line is we don't really know when Stevens was exposed. The hijackers killed on 9/11 could indeed have done it within the margins of certainty on what we do know. Or others who didn't kill themselves but aided the plot could have done it. So don't be so quick to rule out that possibility. Especially when you haven't explained how the first case just happened to show up within a few miles of where the hijackers were staying.