This is an interview with Noam Chomsky about the 9/11 conspiracy theories, he is arguably American foreign policies biggest critic and wouldn't pett to criticise the US government if necessary. Even he denies that it was an inside job, each part is only 3-4 mins so worth a watch.
One of the key things for me is when he mentions how "porous" the system is and that it is hard to keep any information secret (not to mention that everyone in the loop would have to have been ok with it and not have a conscience which seems ridiculously unlikely to me) and also that if the Bush administration were caught "it would be the end of the republican party forever", why in a million years would they risk that, the stakes were miles too high.
This is just one view anyway but an important view i think considering the knowledge and also the political view of the speaker.