Denial,or the simple label of "conspiracy theory" were used for decades to cover up the most blatant operations. But it works less and less now.
Besides that, I discovered that talking about 911 (in USA) is like the virgin Mary. A matter of faith in the Homeland...or not.
IN PLANE SIGHT is one of the best movies on the subject to inject doubt ( at the least) about the most unbelievable series of event in history.