About a week after Sept 11 news reports were released that the terrorist had been buying large blocks of seats to lower the number of passagers on the flights to make them more manageable during the hijackings.
I can't imagine that the airlines couldn't put two and two together with large block of seats being cancelled by MEs.
American Airlines: Amercian Airlines,Good morning how can I help you.
Mr. Terrorist: (w/ thick accent) Yes I would like to cancel my reservations.
AA : What is your flight and how many seats.
MT: Flight 333, we had 240 seats.
AA: 240 seats!, You are going to cancel all of the seats?
MT : Yes, my family has come down with the flu.
AA: That will be find sir. We will credit your VISA card. Thank you for flying with AA.