I have a theory. It's up to you to decide if I am far fetched.
This whole horrible act had to have been planned over several months (if not years)of time. I even believe that these terrorists may have even practiced their steps before the events happened yesterday. By practiced, I mean taken these flight before, snuck weapons (or other equipment that was used to strike terror and control into people) onto those flights.
Now, I question the history of these flights. Have these flights always had so few people on them? I believe so. I also believe that there was a team of at least 3 terrorists on each flight. One to fly the plane and at least 2 for controlling the passengers. Now, assuming I am correct in my assumptions, one has to admit that it is a lot easier for 2-3 terrorists with weapons or supposed bombs to contol a plane that is not full and to control those planes passengers through fear.
The combination of a large plane, full of fuel, and fewer number of people that needed to be controlled led to better opportunities for these terrorists. After all, in any situation, the fewer amount of people that are involed in any situation, the lower the chances for unforseen problem to occur.
I believe that the forth plane that went down in Penn. went down because of passenger interaction. Reports are out that phone calls came in from that forth plane stating that passengers and crew had already been injured and killed. Whether this is true or not, I do not know. But these reports still add validity to my thinking that the passengers sensed that they were going to die. The far stretched possibility also has to exist that the terrorists had to have let the passengers know the plane was going to crash and that their lives would be given for the cause.
It is unlikely that the passengers knew about the WTC and what had happened there, or even the Pentagon. But even if they did, would there be enough heros on this forth plane to overtake the crowd controlling terrorists, and put the plane down to save more lives than would be lost if the target was hit?
I don't think we will ever know? Maybe if the black box is found from the forth plane some questions could be answered.
I realize I have a lot of points here that would take some far fetched circumstances for it all to come together, but I think one important aspect in all of this was that there had to be fewer numbers of passengers on the flights in order for these acts to occur as planned.
God bless the innocent souls!!