Before he killed at least four people in Britain's deadliest attack since the 2005 London bombings, Khalid Masood was considered by intelligence officers to be a criminal who posed little serious threat.
A British-born former teacher and Muslim convert, Masood had shown up on the periphery of previous terrorism investigations that brought him to the attention of Britain's MI5 spy agency.
But he was not under investigation when he sped across Westminster Bridge on Wednesday, plowing down pedestrians with a rented car before running into the parliamentary grounds and fatally stabbing an unarmed policeman. He was shot dead by police.