Sept. 11, 2001, was a life-changing day, even for people who were nowhere near the airline attacks that brought down the twin towers of the World Trade Center and killed nearly 2,800 men and women [source: New York Magazine]. The terror spread as the Pentagon was hit by another plane and news broke that passengers and crew on a fourth, United Flight 93, overtook hijackers and caused the plane to crash into a field, rather than at its intended target [source: History]. Muslim terrorist group al-Qaida claimed responsibility for the attacks. But almost immediately, conspiracy theories started flying around.
Some theorists believed that the U.S. government had advance warning of the attacks, but let them happen anyway. Others claimed that the government actually engineered the attacks, and that al-Qaida wasn't even involved. In fact, some insist that the government created the plot in an attempt to frame Muslims for terrorist activity, justifying war efforts to come in the Middle East [source: What Really Happened]. As evidence, these critics say that the twin towers collapsed too rapidly and in a manner inconsistent with having been hit by a plane, insinuating instead that internally placed explosives were the true culprits for their demise.
However, physicists point out that the architectural design of the towers as "a tube within a tube" made it easy for the towers to be brought down by the energy generated during their collapse, which is estimated to be equivalent to roughly 100 tons (91 metric tons) of TNT per tower. There was also a lack of booming sounds and flashes of light characteristic of explosions [source: Thomas].