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. These include the theory that high-level government officials had advance knowledge of the attacks. Government investigations and independent reviews have rejected these theories. 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.
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. Another prominent belief is that the Pentagon was hit by a missile launched by elements from inside the U.S. government, 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) to advance their geostrategic interests, such as plans to construct a natural gas pipeline through Afghanistan. Other conspiracy theories revolve around authorities having advance knowledge of the attacks and deliberately ignoring or assisting the attackers.
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. 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, but some conspiracy theory groups, including Architects & Engineers for 9/11 Truth, disagree with the arguments made by NIST and Popular Mechanics.
Background
9/11 conspiracy theorists reject one or both of the following facts about the 9/11 attacks:
Al-Qaeda suicide operatives hijacked and crashed United Airlines Flight 175 and American Airlines Flight 11 into the twin towers of the World Trade Center, and crashed American Airlines Flight 77 into the Pentagon. The impact and resulting fires caused the collapse of the Twin Towers and the destruction and damage of other buildings in the World Trade Center complex. The Pentagon was severely damaged by the impact of the airliner and the resulting fire. The hijackers also crashed a fourth plane into a field near Shanksville, Pennsylvania after the passengers and flight crew attempted to regain control of the aircraft. Pre-attack warnings of varying detail of the planned attacks against the United States by al-Qaeda were ignored due to a lack of communication between various law enforcement and intelligence personnel. For the lack of interagency communication, the 9/11 report cited bureaucratic inertia and laws passed in the 1970s to prevent abuses that caused scandals during that era, most notably the Watergate scandal. The report faulted both the Clinton and the Bush administrations with "failure of imagination".
This consensus view is backed by various sources, including: The reports from government investigations – the 9/11 Commission Report (that incorporated intelligence information from the earlier FBI investigation (PENTTBOM) and the Joint Inquiry of 2002), and the studies into building performance carried out by the Federal Emergency Management Agency (FEMA) and the National Institute of Standards and Technology (NIST) Investigations by non-government organizations that support the accepted account – such as those by scientists at Purdue University. Articles supporting these facts and theories appearing in magazines such as Popular Mechanics, Scientific American, and Time. Similar articles in news media throughout the world, including The Times of India, the Canadian Broadcasting Corporation (CBC), the BBC, Le Monde, Deutsche Welle, the Australian Broadcasting Corporation (ABC), and The Chosun Ilbo of South Korea.