A few months after the attacks in US on September 11, 2001, I came across write-ups on the internet which raised serious doubts about the official version of the attacks. These were based on common sense and I was surprised at myself that I didn't myself, earlier, spot the blatant misguiding direction of the official version.
Anyway, after that, things appear reasonably clear to me. Much more has been written about on the internet that raises specific questions on the Sept 11 official theory.
9/11 and the Evidence is the latest write-up that I have come across on the subject.