As far as training hijacking pilots. If you have money and a generation of fanatical individuals to pull from, a few planes, connections to military sources, training a few airforce dropouts to fly a big plane like a top gun cannot be that difficult.
Think about it.
One 3 foot helium weather balloon on a 1,000 foot string tied to a stake on a calm day in a desert. Example; The kid that couldn't pass the math tests for navigation but could fly anything, finds they'll let him fly copilot and train for glory by cutting the string of the ballon, after he learns to over ride automatic flight controls. Give him a buddy that can control the throttles in a 3g banked turn, wave a box cutter menancingly, operate a GPS and knows the New York skyline, it seems that as long as they got on a plane
http://www.scoop.co.nz/stories/HL0301/S00032.htm]
they could hit the building.
I feel there was help with the passengers, perhaps gas canisters planted in place of oxygen a week before in a regular maintenence that had a radio controlled valve could be triggered by a hijacker with a remote that was also carrying a filter specific to the gas. Within minutes all the passengers could be unconscious and within this scenario of a plane, the hijackers could have a key to the cockpit door. Of course the passengers are going to wake up so its up to the hijackers after that to maintain possession of the cockpit.
Considering the explosion
after flight 175 was completely in the building.
where the bomb goes off goes off after the plane is completely inside the building, the bomb must have been shielded from the impact to have a functioning detonator after impact.
It appears that the plane had a bomb aboard not brought on by hijackers. The shielding required would be too heavy or bulky along with the quantity required to make the explosion that was present. This means that the planes were prepared in ways which supports the possibility of gasses being in place to knock out the passengers.