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 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 the 52-year-old was not under investigation when he sped across Westminster Bridge on Wednesday, ploughing down pedestrians with a hired car before running into the parliamentary grounds and fatally stabbing an unarmed policeman.
He was shot dead by police.
Although some of those he was involved with included people suspected of being keen to travel to join jihadi groups overseas, Masood "himself never did so", said a US government source.
"Our investigation focuses on understanding his motivation, his operation and his associates," Britain's senior counterterrorism police officer, Mark Rowley, told reporters.
"Whilst there is still no evidence of further threats, you'll understand our determination is to find out if either he acted totally alone, inspired perhaps by terrorist propaganda, or if others have encouraged, supported or directed him."
Islamic State claimed responsibility for Masood's attack, although it was unclear what links - if any - he had with the militant group. Police said there had been no prior intelligence about his intent to mount an attack.
Born Adrian Russell Ajao in Kent to the southeast of London on Christmas Day in 1964, he moved though several addresses in England, although he was known to have lived recently in Birmingham in central England.
The Daily Mail newspaper said he was brought up by his single mother in the town of Rye on England's south coast, later converting to Islam and changing his name.
Known by a number of aliases, he racked up a string of convictions, but none for terrorism-related offences. His occupation was unclear.
Little detail has officially been given about the man and what might have led him to carry out Wednesday's attack, the deadliest in Britain since the London suicide bombings of 2005 by four young British Islamists, which killed 52.
"Our working assumption is that he was inspired by international terrorism," said Rowley.
Mr Rowley said detectives were questioning nine people in custody, having made two further "significant" arrests in central and northwest England.