London police said that a 75-year-old victim of the Westminster Bridge attack in London has died.

Authorities on Thursday identified a 52-year-old Briton as the man who mowed down pedestrians and stabbed a policeman to death outside Parliament in London on Wednesday, saying he had a long criminal record and once was investigated for extremism - but was not currently on a terrorism watch list.

As millions of Londoners returned to work a day after a rampage that killed four victims and injured at least 30, British Prime Minister Theresa May had a message for other attackers: "We are not afraid."

"Today we meet as normal - as generations have done before us, and as future generations will continue to do," she said to lawmakers' cheers in the House of Commons.

The Islamic State group claimed responsibility for Wednesday's attack, which police said was carried out by Khalid Masood, a U.K.-born resident of the West Midlands in central England. Masood plowed a rented SUV into pedestrians on London's Westminster Bridge, killing an American man and a British woman and injuring more than 30 people of almost a dozen nationalities. He then fatally stabbed a policeman inside the gates of Parliament before being shot to death by an officer.