Every time a conspiracy theory comes up, I always quote the same response, given by a psychologist.
It seems that, as humans, we have trouble grasping the sheer enormity of a tragedy like 911, so one way we assimilate it is to try and find reasons for it, beyond what the actual reasons were.
That means we start to think that it was a government plot, and then we think that the moon landings never happened, and Elvis is still alive, and on and on and on.
It's this simple - if 911 was anything other than what it was - a terrorist atrocity, you would have to swear a seriously large number of people to secrecy, and ensure that every single one of them kept the secret from that day to this.
The only way for three people to keep a secret is if two of them are dead.
Accept that horrible things happen, and there are no hidden plots behind them - just the facts as they are.