The most likely scenario, in my opinion, resembles something along the lines of the manufactured terror entrapment cases the FBI frequently cooks up to trick the American people into thinking there's a terrorism problem when there really isn't. In other words, the 19 hijackers could have easily been unwitting patsies who truly believed they were working on behalf of what they thought were genuine, Muslim fanatics hellbent on striking back at the "Great Satan", but in reality were working on behalf of an organization - Al-Qaeda - set up and covertly funded by the CIA for the primary purpose of conducting false flag terror attacks on demand from client NATO governments (UK, Spain, US, etc...).
This is essentially how wars, conquest, and hegemony are generated in the modern age. Supposedly democratic countries can't just go around invading other countries without a legitimate pretext, because that would be too obviously evil and corrupt. The people in democratic countries generally don't want war, so if their government went around starting a bunch of aggressive wars without good reason, it would quickly become apparent to the people, even the dumbed down, publicly educated ones, that their supposedly democratic doesn't represent them, and that can't be allowed to happen. The illusion of democracy, that the people control their government, must be maintained at all times. The real string pullers - the ruling oligarchs of the international financial coterie - must never be revealed.
That's where a state-sponsored false enemy like Al-Qaeda comes in. Al-Qaeda serves as the much needed bogeyman for the ruling elites. With a fake enemy out there committing fake terror attacks, you can justify aggressive wars, occupations, fascist legislation like the Patriot Act and NDAA, the creation of a full-spectrum police state, consolidation of government power, onerous regulations, drone surveillance, torture, and higher taxes. These are the bread and butter of every would-be tyrant in history, as it's far easier to oppress and exploit someone when they have no recourse against you, legal or otherwise. A system is being put in place that will make it all but impossible for the American people to successfully rebel and alter the course of their government, even if they wanted to. The existence of Al-Qaeda is indispensable to the creation of this system.