I am not sure. I definitely believe our government had the information that Bin Laden was planning an attack and they failed to act on it. Whether they let it happen, were in on it, or caused it I am not sure. To those of you who believe the conspiracy theories like "Loose Change", to get the full story you should read the responses to it:
http://www.popularmechanics.com/science/defense/1227842.html?page=4&c=y
http://www.loosechangeguide.com/LooseChangeGuide.html
They are pretty convincing in debunking the conspiracies. The one thing that doesn't add up to me is the manner in which they fell. If the fires had weakened the steel severely, it most certainly would have affected one side more than the other, rather than heating all steel beam exactly the same. Naturally, it would have weakened the side of the point of impact more. This would make the towers topple to that one side instead of falling straight down. Or, more likely, IMO, the damaged section and above would have just toppled off, leaving the rest standing. Since the building was made to withstand a plane crash, and the lower floors still had full structural integrity, I don't see how it all would have fallen without an inside detonation. Keep in mind that this occured not once, but three separate times, the two WTCs and WTC7. The towers both fell in just one second longer than if they were in a vacuum. And they had thousands of tons of steel, concrete, and glass below them.