A decade after the attacks that killed nearly 3,000 people in New York, Washington and Pennsylvania, the Conspiracy Files looks at why some people still question what really happened on 9/11. Conspiracy theories continue to evolve and now question every aspect of the official account. Why, they ask, was the hole in the Pentagon so small? Why did the World Trade Centre buildings collapse as if being demolished by explosives? Why did one skyscraper fall when it was never hit by a plane? And why was the world's greatest military power so unprepared and so slow to react when warnings had been received? The death of Osama Bin Laden might have been expected to put an end to the conspiracy theories, but the failure to release any pictures of Bin Laden's death and the hasty disposal of his body in the Arabian Sea, has instead given these theories a new burst of life. Featuring key witnesses, CIA and FBI interviewees and leading sceptics, the programme analyses the evidence and looks at what makes conspiracy theories so persistent and so powerful.