A British-born convert to Islam, 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, plowing 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 U.S. government source, who spoke to Reuters on condition of anonymity.

Britain's senior counter-terrorism police officer, Mark Rowley, told reporters: "Our investigation focuses on understanding his motivation, his operation and his associates."

He added: "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.

"An act of terrorism tried to silence our democracy," Prime Minister Theresa May told parliament. "He took out his rage indiscriminately."

BRITISH-BORN KILLER

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. Other media reports said he was a married father of three and a former English teacher who was into bodybuilding.

One soccer team photograph of Masood, taken at school in southern England, showed the future attacker smiling.

Little detail has been given by the British police 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.

Known by a number of aliases, he racked up a string of convictions, but none for terrorism-related offences. His occupation was unclear.

It was as long ago as November 1983 that he first came to the attention of authorities when he was found guilty of causing criminal damage. His last conviction came 14 years ago in December 2003 for possession of a knife.

He may have taught in Saudi Arabia for four years from 2005 but there was no confirmation of this. A spokesman for the Saudi interior ministry referred Reuters questions to the British authorities.

"Our working assumption is that he was inspired by international terrorism," said Rowley.

But Masood's age does not fit the profile of militant attackers, who are typically younger than 30, according to counter-terrorism officers.

Rowley said detectives were questioning nine people in custody, having made two further "significant" arrests in central and northwest England.

Iwona Romek, a former neighbor from Birmingham, told reporters: "When I saw the pictures on TV and in the papers of the man who carried out the attack, I recognized him as the man who used to live next door.

"He had a young child, who I'd think was about 5 or 6 years old. There was a woman living there with him, an Asian woman. He seemed to be quite nice, he would be taking care of his garden and the weeds."

In December, she said, he suddenly moved out.

BIRMINGHAM CONNECTION

Birmingham has been one of the hotbeds for British Islamists. According to a study by the Henry Jackson think tank earlier this month, 39 of 269 people convicted in Britain of terrorism offences from 1998 to 2015 came from the city.

Among those plots was one to kidnap and behead a British soldier. In December, two men were found guilty of planning to give 3,000 pounds ($3,750) to Brussels bombing suspect Mohamed Abrini - widely known as "the man in the hat".

There are over 213,000 Muslims in Birmingham, making up over a fifth of the population, according to the 2011 census, and there has been growing concern about divisions in the diverse city.

"It has been disturbing today to learn of the apparent Birmingham connection to this atrocity," said the Birmingham Faith Leaders Group, made up of representatives of major religions from the city. "We implore people to recognize that such actions are taken by individuals, not by whole communities."

The car Masood used in Wednesday's attack had been hired in Birmingham from rental firm Enterprise, suggesting he still had connections to the area.

Since the attack, police have raided a number of addresses across the city, arresting five men and two women on suspicion of preparing terrorist acts.

Masood may have rented an apartment close to the Edgbaston area of the city, not far from the Enterprise offices, and that was one of the properties raided by armed officers.

On the eve of the attack that May cast as an assault on democracy, Masood spent his last night in a budget hotel in Brighton on the south coast, where he ate a takeaway kebab, the Sun newspaper said.

Michael Petersen, a guest who saw him at the hotel reception, said Masood appeared polite and had done nothing to arouse suspicion.

"Nothing in his demeanor or his looks would have given me any thoughts that would make me think he was anything but normal," he said.