The theory: Al Qaeda was not (or was not solely) responsible for the 9/11 attacks. The U.S. government either allowed the attacks to happen or orchestrated them.
The details: Dubious theories about what really occurred on Sept. 11, 2001, began to circulate almost immediately after the attacks took place. For instance, one Arab satellite network reported that 4,000 Jews had skipped work on the day of the attacks. Several books released in the following months in Europe and the Middle East postulated that the attacks had been a "false-flag" operation orchestrated by neoconservatives within the U.S. government to drum up support for the war in Iraq.
It's hardly surprising that such misinformation would spread in the wake of such a catastrophic event. What's more shocking is that a recent poll of 17 countries showed that a majority of people still aren't convinced that Al Qaeda was responsible.
Many "truthers" take issue with other aspects of the official explanation. The popular 2005 documentary "Loose Change" argues that airliners could not have caused the twin towers to collapse in the way they did and that the lack of debris at the Pentagon and Flight 93 crash sites is inconsistent with what a destroyed jet would create. Popular Mechanics devoted an entire special issue to debunking the theories but "Loose Change" remains as popular as ever, with a host of sequels and spinoffs proliferating online.
The "truther" controversy hit the Obama administration this month when "climate czar" Van Jones was forced to resign after it was revealed that he had signed a petition calling for further investigation into who was behind the 9/11 attacks. Other signatories included erstwhile presidential candidate Ralph Nader, historian Howard Zinn, and Congresswoman Cynthia McKinney. Actor Charlie Sheen has also emerged as a prominent advocate for the "truther" movement.
The truth: The bipartisan 9/11 Commission thoroughly investigated the events of September 11 and found conclusively that Al Qaeda was responsible for the attacks. Osama bin Laden himself claimed responsibility for the attacks in 2004, saying they were in retaliation for U.S. policies in the Middle East.