FROM THE MOMENT the first airplane crashed into the World Trade Center on the morning of September 11, 2001, the world has asked one simple and compelling question: How could it happen?

Three and a half years later, not everyone is convinced we know the truth. Go to Google.com, type in the search phrase "World Trade Center conspiracy" and you'll get links to an estimated 628,000 Web sites. More than 3000 books on 9/11 have been published; many of them reject the official consensus that hijackers associated with Osama bin Laden and Al Qaeda flew passenger planes into U.S. landmarks.

Healthy skepticism, it seems, has curdled into paranoia. Wild conspiracy tales are peddled daily on the Internet, talk radio and in other media. Blurry photos, quotes taken out of context and sketchy eyewitness accounts have inspired a slew of elaborate theories: The Pentagon was struck by a missile; the World Trade Center was razed by demolition-style bombs; Flight 93 was shot down by a mysterious white jet. As outlandish as these claims may sound, they are increasingly accepted abroad and among extremists here in the United States.

To investigate 16 of the most prevalent claims made by conspiracy theorists, POPULAR MECHANICS assembled a team of nine researchers and reporters who, together with PM editors, consulted more than 70 professionals in fields that form the core content of this magazine, including aviation, engineering and the military.

In the end, we were able to debunk each of these assertions with hard evidence and a healthy dose of common sense. We learned that a few theories are based on something as innocent as a reporting error on that chaotic day. Others are the byproducts of cynical imaginations that aim to inject suspicion and animosity into public debate. Only by confronting such poisonous claims with irrefutable facts can we understand what really happened on a day that is forever seared into world history.--THE EDITORS

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