10 ARRESTED, 21 SITES SEARCHED IN MASSIVE PROBE AFTER LONDON ATTACK

British police are combing through “massive amounts of computer data,” have searched 21 sites and have contacted thousands of witnesses in a vast operation to trace how a British man became radicalized and launched a deadly attack on Parliament, a senior official said Friday.

In a briefing outside Scotland Yard, London’s top counterterror officer, Mark Rowley, said more “significant” arrests had been made, bringing to 10 the number of people in custody over Wednesday’s attack, which killed four people and the assailant.

Police said the attacker, Khalid Masood, was born Adrian Russell Ajao in southern England in 1964. He was also known as Adrian Elms and “may also be known by a number of other names,” police said.

The latest arrests were a man and a woman detained early Friday in Manchester, northwest England. Police believe Masood acted alone but Rowley said police were trying to determine whether others “encouraged, supported or directed him.”

The Islamic State of Iraq and the Levant has claimed responsibility for the attack on Westminster Bridge and at Parliament.

Detectives have searched 21 properties in London, Brighton, Wales, Manchester and the central English city of Birmingham in one of Britain’s biggest counterterrorism operations in years.