Khalid Masood, once named Adrian Russell Ajao, used rental car to mow down pedestrians

Police are combing through "massive amounts of computer data" and have contacted thousands of witnesses as they look for clues about why a British-born man became radicalized and launched a deadly attack on Parliament, a senior police official said Friday.

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• U.K. attacker was British-born man once investigated for 'violent extremism'

As the fast-moving investigation led to more arrests and searches, police revealed that the attacker, Khalid Masood, was born Adrian Russell Ajao in southern England in 1964. Police said he hadalso gone by the name Adrian Elms, and could be known by a number of other names, too.

In a briefing outside Scotland Yard, London's top counterterror officer, Mark Rowley, said two more "significant" arrests had been made, bringing to nine the number of people in custody over Wednesday's attack.

He appealed to the public for information about Masood.

Detectives have searched 21 properties in London, the central English city of Birmingham and Wales.

"We've seized 2,700 items from these searches, including massive amounts of computer data for us to work through," Rowley said, adding that contact had been made with 3,500 witnesses.

"We've received hundreds of uploads of video images to our online platform. Given this attack was in the heart of the capital, we also, of course, are dealing with statements from a wide range of nationalities."

Masood drove his car into crowds of people on Westminster Bridge on Wednesday afternoon before fatally stabbing a police officer at the Houses of Parliament. He was shot dead by police.

An American man from Utah, a British man and British woman were killed on the bridge, and police officer Keith Palmer died at Parliament, police said.

The latest victim, who died in a hospital on Thursday, was identified as 75-year-old Leslie Rhodes from south London.

More than 50 people of a dozen nationalities were injured, 31 of whom required hospital treatment.

"Those affected include a real cross-section of ages from at least 12 nationalities," Rowley said. "It's a poignant reminder, I think, that the impact of this attack on the capital will reach around the world. "

Rowley said two police officers targeted in the attack have significant injuries. Two other people also remain in critical condition, one with life-threatening injuries.

Convictions between1983 and 2005

The 52-year-old attacker was born in southeastern England and had most recently been living in Birmingham, where several properties have been searched by police.

Police say Masood has used several aliases and had a string of convictions between 1983 and 2003 for offences, including assault and possession of an offensive weapon.

Prime Minister Theresa May said Thursday that Masood was "investigated in relation to concerns about violent extremism" some years ago. But she called him "a peripheral figure."

On Thursday, ISIS, the Islamic State in Iraq and Syria,claimed responsibility forMasood'sattack, although it was unclear what linksif anyhe had with the militant group.

Police said there had been no prior intelligence about his intent to mount an attack. And they have offered little detail about him and what might have led to the attack.

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

9 in custody

Nine people remain in custody on suspicion of preparing terrorist acts.

Once his identity became known, police and the media began to trace the movements of the attacker in his final hours.

He was normal, in fact friendly, because we spent possibly five or 10 minutes talking to him about his background and where he came from- Sabeur Toumi, hotel manager

The carMasoodused in the attack had been rented from rental firm Enterprise's Spring Hill branch in Birmingham.

The manager of a hotel in the beachside city of Brighton, where Masoodstayed the night before the incident, said he seemed unusually outgoing and mentioned details about his family, including having a sick father.

"He was normal, in fact friendly, because we spent possibly five or 10 minutes talking to him about his background and where he came from," Sabeur Toumi told Sky News on Friday.

Police raided the room at the Preston Park Hotel in Brighton after the attack, searching for clues about Masood. Among the items seized were the trouser press and the toilet paper holder.

A former neighbour from Birmingham said: "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 five or sixyears 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,"IwonaRomek, 45, told reporters at her home.

In December, she said, he suddenly moved out.