Nearly twenty years ago, on 11 September 2001, four passenger aircraft were hijacked and flown into the World Trade Center in New York, the Pentagon near Washington, and a field in Shanksville, Pennsylvania. Nearly three thousand people were killed. The narrative in the weeks and months that followed seemed straightforward: the attacks had been masterminded by al Qaeda leader, Osama bin Laden, an embittered terrorist with an abiding hatred of the West. But, as the twenty-year anniversary approaches, that neat explanation still fails to answer some important questions surrounding that fateful day.