When national tragedies occur they naturally trigger painful soul-searching. Whether it is the murder of JFK or the dreadful events of 9/11, Americans have searched long and hard for answers. In both cases a national consensus emerged over time about the single gunman who shot JFK... and the reasons the towers collapsed on 9/11. But some still question the so-called 'official version' of history. They passionately believe in other - often radical - explanations. And they think there might be conspiracies to hide the real truth.