Who was responsible for 9/11, and how do you know?

So, 9/11 conspiracy theorists, who call themselves 9/11-truthers, seem to be a significant presence on the internet. (For the record, I believe the “mainstream” interpretation that al-Qaeda or allied terrorists and their network were solely responsible and the Bush administration was unaware of that specific threat).

This isn’t a “poll” thread—I’d like your responses to be written like arguments or short essays, explaining why you believe what you do and how you can support your position. Obviously you can answer however you want, but let’s all try to have a civil and well-thought out discussion about it.