My guess is that the attacks were a combination of Islamic fundamentalist conspiracy, gross governmental negligence, and insurance fraud/capital murder. It is likely that Al Qaeda planned and executed the 9/11 attacks but it is also likely that US/Egyptian/Turkish/Israeli/etc… intelligence discovered the plot beforehand. Perhaps the White House took no action to stop the attack knowing that if the plot succeeded they would have justification to invade Iraq. Also, WTC leaseholder Larry Silverstein is a rich and powerful guy who might have some connections in intelligence circles; he could have hired a foreign demolition team of 4 or 5 men to plant the thermite cutting charges in his buildings (they don’t use “bombs” in building demolition) so that he wouldn’t be stuck with an expensive asbestos-filled white elephant afterwards.
We’ll probably never really know what happened. However, Operation Northwoods is proof that the US has at least planned false flag attacks in the past. The fact that CIA operative Luis Posada remains free in the US after executing the bombing of Cubana Flight 455 in which 73 people died shows that US not only deals with known terrorists, but actually protects them. And the USS Liberty incident is proof that even our closest allies have been guilty of treachery against us when acting in their national interest. As I wrote earlier, 9/11 will be the Kennedy assassination of our generation, a hall of mirrors in which the truth will remain forever hidden.