9/11 conspiracy theories

Various conspiracy theories 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 is evidence that was ignored, concealed, or overlooked. Most of these theories either lack evidence or have been debunked by experts.

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 commercial airliners were allowed to hit the targets via an effective stand-down of the U.S. military. Motives conspiracy theorists ascribe to such actions include justifying the U.S. invasions of Afghanistan in 2001 and Iraq in 2003 (even though the U.S. government concluded Iraq was not involved in the attacks) to advance U.S. 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 investigated and rejected these theories.