Photo: The Associated Press

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.

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.

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.

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.

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."