Even if theoretically the terrorists could have hi-jacked 4 planes, and have been able to fly them to their selected targets, how did these 19 people get past the billion dollar air defence system of America? Where was NORAD the million dollar system that can pick out any planes flight number? And where was the fighter jets?
But this lack of defence can be explained without including the need of a conspiracy. It’s still not good for the American government, especially the secretary of defence.
NORAD is designed to pick out enemy planes outside of America, the radar going in a kind of a doughnut shape around the US. This means that if an attack came from within, this utility would be practically useless. Normal radar would also be helpless since the transponders where removed whilst in the air, meaning the security would have had to search through hundreds of of identical bleeps on the radar screens to find the hi-jacked planes.
They did eventually find flight 146, half an hour after the first tower attack, and sent 2 of the 14 fighter jets on standby to intercept the plane. They got there too late, the reason being at the time it was against the law to travel at subsonic speeds above heavily populated areas.