Nineteen men armed only with box-cutters and their fanaticism successfully hatched a plot totally unnoticed by America's $40bn a year intelligence-gathering machine.
They succeeded because they lived and worked, not in the shadows where spies operate, but in full view.
In fact, one of the most bizarre ironies of all this is that five of the hijackers lived in a motel right outside the gates of the NSA.
Early on the morning of 11 September, when Hani Hanjour and his four accomplices left the Valencia Motel on US route 1 on their way to Washington's Dulles airport, they joined the stream of NSA employees heading to work.
Three hours later, they had turned flight 77 around and slammed it into the Pentagon.