A variety of conspiracy theories question the mainstream account of the September 11, 2001 attacks in the United States. These theories assert that the official report on the events is not sufficiently forthcoming, thorough or truthful. Many critics allege that individuals in the government of the United States knew of the impending attacks and intentionally failed to act on that knowledge. Some critics state that the attacks could have been a false flag operation carried out by high-level officials in the U.S. government who may have engaged compartmentalization to keep knowledge of their actions limited. The common suspected motives were the use of the attacks as a pretext to justify overseas wars, to facilitate increased military spending, and to restrict domestic civil liberties.

Many of the theories have been voiced by members of the 9/11 Truth Movement,[1] a name adopted by some organizations and individuals who question the mainstream account of the attacks. Some 9/11 Truth Movement members question the accuracy of the mainstream account of the attacks, and they are committed to further investigation. Others claim that the collapse of the World Trade Center was the result of a controlled demolition and/or that United Airlines Flight 93 was shot down.[2] Some also contend that a commercial airliner did not crash into the Pentagon; this position is debated within the Truth Movement, many of whom believe that AA Flight 77 did crash there, but that it was allowed to do so via an effective stand down of the military.[3]

Published reports by the US National Institute of Standards and Technology rejected the controlled demolition hypothesis.[4][5] The community of civil engineers generally accepts the mainstream account that the impacts of jets at high speeds in combination with subsequent fires, rather than controlled demolition, led to the collapse of the Twin Towers.[6]