Oh, and BTW, they lied about weapons of mass destruction. They lied about Saddam's support of al Quida. In fact they lied about every excuse they used to invade Iraq. So, why would they tell you the truth about who was responsible for 911? Was that not the main reason they invaded Iraq and Afghanistan? They lied, you bought it.
The funniest thing about conspiracy enthusiasts is that they claim the government is capable of secretly carrying out 9/11 (a task that would be so complex as to be utterly mind boggling) but are so incompetent that they couldn't smuggle in and plant a few WMD's in Iraq. They'd have a little more credibility if at least their premise was logically consistent.