9/11 conspiracy theories allege that the September 11 attacks in 2001 were either intentionally allowed to happen or were a false flag operation orchestrated by elements within the United States government. The most prominent claim is that the collapse of the World Trade Center and 7 World Trade Center were the result of a controlled demolition rather than structural weakening due to fire. Another prominent claim is that the Pentagon was hit by a missile launched by elements from inside the U.S. government or that a commercial airliner was allowed to do so via an effective stand down of the American military. The common prescribed motives are the use of the attacks to justify the invasion of Afghanistan and Iraq, to facilitate increased military spending, and to restrict domestic civil liberties.

Published reports and articles by the U.S. National Institute of Standards and Technology, Popular Mechanics and mainstream media have rejected the 9/11 conspiracy theories. Civil engineers state that the impacts of jet aircraft at high speeds in combination with subsequent fires, rather than controlled demolition, led to the collapse of the Twin Towers and WTC 7.

Foreknowledge

It has been claimed that action or inaction by U.S. officials with foreknowledge was intended to ensure that the attacks took place successfully. For example, Michael Meacher, former British environment minister and member of Tony Blair's Cabinet until June 2003 claims that the United States knowingly failed to prevent the attacks.Author David Ray Griffin alleges that the 9/11 conspiracy was considerably larger than the government claims and that the entire 9/11 Commission Report "is constructed in support of one big lie: that the official story about 9/11 is true."

One popular conspiracy theory suggests there was a suspiciously high volume of put options placed on United Airlines and American Airlines stocks just before 9/11. According to this theory, trading insiders knew in advance of the coming events of 9/11 and placed their bets accordingly. While this may look suspicious in isolation, the general volume of put trading on these stocks reached similar levels at earlier points in the year. In fact, American Airlines had just released a major warning about possible losses.

Another common claim is that the North American Aerospace Defense Command (NORAD) issued a stand down order or deliberately scrambled fighters late to allow the hijacked airplanes to reach their targets without interference. According to this theory, NORAD had the capability of locating and intercepting planes on 9/11, and its failure to do so indicates a government conspiracy to allow the attacks to occur. The Web site emperors-clothes.com argues that the U.S. military failed to do their job. StandDown.net's Mark R. Elsis says "There is only one explanation for this.... Our Air Force was ordered to Stand Down on 9/11."

In September 2001, NORAD generals said they learned of the hijackings in time to scramble fighter jets. Later, the U.S. government released tapes claiming to show the Federal Aviation Agency (FAA) did not tell the military about the hijackings until three of the four planes had crashed, a fact that would indicate that the FAA repeatedly lied to other U.S. government agencies.

Phil Mole of Skeptic magazine has explained that it is neither quick nor easy to locate and intercept a plane behaving erratically, and that the hijackers turned off or disabled the onboard radar transponders. Without these transponder signals to identify the airplanes, the hijacked airplanes would have been only blips among 4,500 other blips on NORAD’S radar screens, making them very difficult to track.

According to Popular Mechanics, in fact, only 14 fighter jets were on alert in the contiguous 48 states on 9/11. There was no automated method for the civilian air traffic controllers to alert NORAD. A passenger airline hadn't been hijacked in the US since 1979. "They had to pick up the phone and literally dial us," says Maj. Douglas Martin, public affairs officer for NORAD. According to Popular Mechanics, "In the decade before 9/11, NORAD intercepted only one civilian plane over North America: golfer Payne Stewart's Learjet, in October 1999." With passengers and crew unconscious from cabin decompression, the plane lost radio contact but remained in transponder contact until it crashed. Even so, it took an F-16 22 minutes to reach the stricken jet from the time when contact was lost.

Rules in effect back then, and on 9/11, barred supersonic flight on intercepts. Before 9/11, all other NORAD interceptions were limited to offshore Air Defense Identification Zones (ADIZ). "Until 9/11 there was no domestic ADIZ," says FAA spokesman Bill Schumann. After 9/11, the FAA and NORAD increased cooperation. They set up hotlines between command centers while NORAD increased its fighter coverage and installed radar to watch airspace over the continent.