The events of September 11, 2001 are the defining rationale for the actions of the US government in the ensuing years, yet few have logically and scientifically studied what actually happened on that day, and the events that unfolded in reponse.
If you are not among those few, I recommend that you begin by reading David Ray Griffin’s books The New Pearl Harbor and The 9/11 Report: Errors and Omissions. Griffin is a retired professor of theology, not a wild-eyed radical by any stretch of the imagination. He collects facts, such as the standard Air Force procedure in response to an off-course air craft, the temperature at which steel melts, and the policies of insurance companies relative to fires in steel frame buildings. He considers a number of the possible interpretations of the facts (the Bush administration had no idea, they knew and didn’t prevent it, they executed it for their own benefit, etc.) and then considers how each interpretation could exist logically in the real world.
In addition, the Internet is rich with sites and documentary footage on this subject, which you can access by googling 9/11 Truth.