A reasonable question, but upon reflection there are others, probably more apropos:
Where did the flight come from last?
Was there time to make a thorough search of the aircraft in question?
Who had access to the aircraft in between flights? Are all the individuals known and accounted for?
Were the flights cancelled before the passengers were on board? This can be important, if it is possible some of the perps are on the flight. They should all be allowed to board and then requested to disembark and be individually videotaped on exiting. I am of the opinion that all flights ordered to land on 911 should have had every passenger videotaped for future identification.
Presumably, every crew member has had a background check. In addition, I believe strongly that every potential passenger on any airliner has the absolute right to know if a muslim crewmember is aboard, and decide to use the aircraft or not. The reasoning is complex but real, and, for me, not negotiable. Support for terrorists by acquiescence is real and massive among muslims, and until they themselves address that reality, I need to make my own judgements as to whom I fly with.