There are various conspiracy theories that attribute the preparation and execution of the September 11 attacks against the United States to parties other than, or in addition to, al-Qaeda.[1] These include the theory that high-level government officials had advance knowledge of the attacks. Government investigations and independent reviews have rejected these theories.[2][3] Proponents of these theories assert that there are inconsistencies in the commonly accepted version, or that there exists evidence that was ignored, concealed, or overlooked.[4]
The most prominent conspiracy theory is that the collapse of the Twin Towers and 7 World Trade Center were the result of controlled demolitions rather than structural failure due to impact and fire.[5][6] Another prominent belief is that the Pentagon was hit by a missile launched by elements from inside the U.S. government,[7][8][9] or that hijacked planes were remotely controlled, or that a commercial airliner was allowed to do so via an effective stand-down of the American military. Possible motives claimed by conspiracy theorists for such actions include justifying the invasions of Afghanistan and Iraq (even though the U.S. government concluded Iraq was not involved in the attacks)[10] to advance their geostrategic interests, such as plans to construct a natural gas pipeline through Afghanistan.[11] Other conspiracy theories revolve around authorities having advance knowledge of the attacks and deliberately ignoring or assisting the attackers.[4][12][13]
The National Institute of Standards and Technology (NIST) and the technology magazine Popular Mechanics have investigated and rejected the claims made by 9/11 conspiracy theorists.[14][15][16] The 9/11 Commission and most of the civil engineering community accept that the impacts of jet aircraft at high speeds in combination with subsequent fires, not controlled demolition, led to the collapse of the Twin Towers,[17][18] but some conspiracy theory groups, including Architects & Engineers for 9/11 Truth, disagree with the arguments made by NIST and Popular Mechanics.[19][20]
Since the attacks, a variety of conspiracy theories have been put forward in websites, books and films. Many groups and individuals advocating 9/11 conspiracy theories identify as part of the 9/11 Truth movement.[34][35][36] Within six hours of the attack, a suggestion appeared on an Internet chat room suggesting that the collapse of the towers looked like an act of controlled demolition. "If, in a few days, not one official has mentioned anything about the controlled demolition part," the author wrote, "I think we have a REALLY serious problem."[37] The first theories that emerged focused primarily on various perceived anomalies in the publicly available evidence, and proponents later developed more specific theories about an alleged plot.[11] One false allegation that was widely circulated by e-mail and on the Web is that not a single Jew had been killed in the attack and that therefore the attacks must have been the work of the Mossad, not Islamic terrorists.[11]