Twenty years ago 19 men hijacked four airliners and changed the world. While we remember the victims and count the costs of an operation that redefined terrorism, we look at how the US found itself so open to the brutal attacks that took place on 9/11. The US has the largest, most sophisticated and best funded intelligence community in the world, but it seems bigger isn’t necessarily better, and we look at how the CIA and FBI missed the clues to the moment that defined the first two decades of the 21st century.