It's really something horrible; I couldn't believe it this afternoon when a colleague told me about it.
All the condolences to the relatives of the victims.
There must have been at least 2 terrorists per hijacked plane; I think all were interior flights. How difficult is it to get into a plane with a weapon? I remember the control was much less tough than with international flights. There, you simply can't get into a plane with something metallic in a pocket, without being noticed.
Larry is right: only a few terrorists are necessary (in order to keep it quiet).
The terrorists were certainly in control of the planes (the Towers were very narrow targets if you fly into them at 400 mph), but how was it possible? If no weapons were involved, how could the pilot and copilot have let this happen?
So the terrorists must have been armed. Did the crew realize the determination of the terrorists?
Will we ever know? Maybe the black boxes will help.
Most probably, the plane that went down in Pennsylvania had an heroic crew.
It's too early to answer many questions, but it won't last long.

Raoul