I have my own 9/11 theory. There's this guy, see? And he leads a cult and manages to get tons of followers. He convinces them that America (more specifically the U.S.) is evil and is the source of all evil in the world so he talks his followers into doing bad things to anything U.S. related. Convinced that money is the root of all evil and that the World Trade Center (heretofore called the Twin Towers, the name I grew up with) is the center of all money-making endeavors he decides that they must be toppled. First he blows up a bomb in the basement but it wasn't enough. So he comes up with an evil plot to hijack some big planes and literally FLY them, full speed, into the towers. So he gets a bunch of fanatical followers together and trains them to fly planes. The plan was to hijack dozens of planes and fly them into all kinds of symbols of the evil U.S. but several of his "crews" chicken out and others were foiled by the swift response of grounding all the planes, but I get ahead of myself.
Knowing that security is lacking but still enough to prevent guns and bombs they take innocent knives aboard and with these knives cut up, kill, and otherwise disable the flight crew and convince the passengers that it's just a run-of-the-mill hijacking. They then fly these planes into the Twin Towers at full speed, causing enough damage that their structual integrity is compromised and they both collaspe within hours. He also manages to plunge one right into the Pentagon. The passengers of another, after hearing of the first three over their cell phones, decide to fight back but that plane crashes as a fight ensues in the flight deck. There were possibly more planes destined for destruction but this cult leader greatly underestimates the response of the U.S. and he and many of his followers were surprised that all planes were very quickly grounded over the U.S. and much of Canada and the rest of the world.
This cult leader, then smiling that he at least succeeded in bringing down the Twin Towers, hides out in a cave and keeps trying to do bad things to all things American (more specifically, all things from the U.S.). One day we will find him, or his bones, but until that day we cannot prove the existence of this leader. Until we find him, this is all still theory...