I seem to recall stories soon after 9/11 that aircraft that were rerouted during the immediate grounding of all aircraft had suspicious passengers deplane. I recall a story where a plane was diverted to a midwestern airport (St Louis maybe) and small groups of foreign passengers exited and disappeared. I’m sure there were more aircraft that were to be sequenced and timed to crash into more targets like the Capitol, White House, Pentagon, etc.
I believe this was part of the "train Arabs" story - the men boarded an Amtrak train in St. Louis and were found to be carrying disguises, etc.
I just tried to Google "Train Arabs" and the first 50 hits are about persecution of Muslims by white people all over the world, but especially in the US.
I don't have time to look further, but it was a big story for a few days.
Anyone have a link or remember details?