It was also in the 9-11 commission report, I believe, the fact that the US moved to shut down all the airports on 9-11 in North America prevented a second wave of attacks that had been planned
I have a friend who is a border guard at a primo area for terrorists to target and he claims they were told about a potential second wave that day
I'm glad Moussaoui was found and arrested in part thanks to a "head's up" flight school.