I recently discovered to my horror that one of my brothers is a 9-11 truther. I spent a lot of time talking to him about the process of preparing a controlled demolition of a building and how buildings collapse under such a process as opposed to how the twin towers collapsed. I also spent a lot of time showing him photos and testimony of plane wreckage at the Pentagon, to refute the theory that the building had been hit by a missile. The evidence I gave him had utterly no effect. Trying to talk to him made me appalled that we in this country can't even agree on relatively easily accessible facts anymore.
Al Qaeda attacked the U.S. on 9/11/01. The U.S. government had no bigger role in this specific attack than underestimating lots of cues that something like it would happen, and not keeping a close enough watch on who was in the country. That's what the facts establish. And that's already bad enough. No conspiracy theories necessary or true.
It's the same crap with Holocaust deniers. They believe what they believe because of some underlying prejudice or preconceived hatred. When you show Holocaust deniers the unlimited truths of the disaster, they make excuses for why the proof is flawed and point to tiny inconsistencies from one or two people.