Conspiracy theories have proliferated following the attacks in the US on 11 September 2001. An opinion poll ... for BBC's The Conspiracy Files in 2011, found that 14% of people questioned in the UK and 15% in the US did not believe the official explanation that al-Qaeda was responsible, and instead believed the US government was involved in a wider conspiracy. Among 16 to 24-year-olds that belief rises to around one in four. Ten years on from the attacks [conspiracy theories] now question every aspect of the official account. The starting point for 9/11 conspiracies is that many people find it hard to believe 19 young men, armed with just knives and box-cutters, could casually walk through airport security, hijack four commercial planes and then within the space of 77 minutes destroy three of the iconic symbols of America's power, in the face of the world's most powerful and technologically-advanced military superpower. It is a similar argument that questions whether a lone gunman could have killed President John F Kennedy, then the most powerful and best-protected man on the earth, or how someone so special as Princess Diana could die in a car crash. We don't know the full story of exactly what happened," says American radio talk show host Alex Jones. "It needs to be investigated."