The events of 9/11 seem very clear to most people in the world. Nineteen terrorists were able to get through airport security, hijack four passenger airliners and fly them into targets to cause maximum devastation to New York and Washington D.C.

However to some, the reaction of the United States in the aftermath of 9/11 does not add up, raising suspicions of an "inside job."

The most common conspiracy theory is the anti-Jewish theory born out of the attacks on the World Trade Center on 9/11. Anti-Semite conspiracy theories are more widespread in the Muslim world than the western world in 2011. There is a theory among the Islamic fundamentalists across the Muslim world that 9/11 was part "Zionist" plan to take over the world, a theory that is widespread amongst fundamentalists in Iran.

Ahmad Yousef, Editor-in-Chief of the Washington based Middle East Magazine, said on Al Manar TV, in December 2004: "Israel is capable of penetrating certain Islamic circles, of directing and running them behind the scenes, so that they conduct operations from which Israel benefits. Anyone who considers the events of 9/11 cannot say that the Muslims gained anything."

Iranian channel Jaam-e Jam TV 1 claimed on June 1, 2004: "A while afterward, a source in American military intelligence raised details pertaining to an intelligence memo regarding Israel's espionage organization, the Mossad, and its role in the events of September 11. In fact, the claim that Israel was involved in the blast of September 11, and used it as a basis of America's new strategy for fighting the world of Islam, disappeared in the media coverage -- but world public opinion still believes this possibility."

In 2008 researchers with the Programme of International Policy Attitudes spoke with people in 21 countries around the world asking who they believed were behind the attacks on America on 9/11, 46 per cent of those surveyed said al Qaeda was responsible, 15 per cent said the U.S. government, 7 per cent said Israel and 7 per cent said some other perpetrator. One in four people said they did not know who was behind the attacks.

A Scripps-Howard from 2007 found that 37 per cent of respondents believed that it was "very likely" that some people in the U.S. federal government had specific warnings of the 9/11 attacks.