Khalid Sheikh Mohammed: How '9/11 mastermind' slipped through FBI's fingers

The man accused of hatching the devastating plot to fly hijacked passenger planes into US landmarks 20 years ago is locked up awaiting trial. But could he have been stopped years before?

"He was my guy."

Frank Pellegrino was sitting in a hotel room in Malaysia when he saw the television pictures of the planes crashing into the Twin Towers. The first thing he thought was: "My God, it's got to be Khalid Sheikh Mohammed."

The target and the ambitions were a match and Pellegrino was in a unique position to know.

The former FBI special agent had pursued Mohammed for nearly three decades, yet the alleged 9/11 mastermind is yet to face justice.

A lawyer for Mohammed has told the BBC it may be another 20 years before the case is concluded.