Reeperette made a very valid point, one I've been mulling over during the day.
In fairness to the passengers, It's likely they never knew what the hijackers had in store until is was too late. The terrorists are unlikely to have informed the passengers of the plan, knowing well that rather than be used to kill their own people in large quantities, they would likely have fought back with mortal force, overpowering them with sheer numbers.
Letting slip that they were planning what they were would have made controlling the plane and its' occupants completely impossible. As far as anyone was concerned, they'd take over the plane, order it to be flown to somewhere, and then make demands. The only people who are likely to have seen the flaw in this thinking would have been the crew and attendants, who would have well known that their flights were not equipped/fuelled to fly to far off destinations, being on internal routes. For this reason I'd say they were eliminated pretty quickly.
Incidently, Friday has been declared an official day of mourning here, as a gesture of solidarity with the American people.