LONDON — Authorities on Thursday identified a 52-year-old Briton as the man who mowed down pedestrians and stabbed a policeman to death outside Parliament, 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, also known as ISIS, 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 a Utah man and a British woman and injuring 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.

A 75-year-old victim on the bridge died Thursday of his wounds, police said.

Vincenzo Mangiacarpe, an Italian boxer visiting Parliament, said he saw the attacker get out of the car wielding two knives.

“You can imagine if someone was playing a drum on your back with two knives — he gave (the policeman) around 10 stabs in the back,” Mangiacarpe said.

The dead were identified as Kurt Cochran, 54, of Utah and British school administrator Aysha Frade, 43 — both struck on the bridge — and 48-year-old Constable Keith Palmer, a 15-year veteran of the Metropolitan Police. The 75-year-old victim was not identified.

Police arrested eight people — three women and five men — on suspicion of preparing terrorist acts as authorities sought Masood's motive and possible support network. One arrest was in London, while the others were in Birmingham. Police said they were searching properties in Birmingham, London and Wales.

Masood's convictions between 1983 and 2003 included assault, weapons possession and public order offenses, London police said.

But he “was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack,” police added.

Many suspects in British terrorist attacks and plots have had roots in Birmingham, England's second-largest city, and several local mosques have been linked to extremist clerics.

A home raided in Birmingham was one where Masood lived until late last year, a neighbor said.

Neighbor Iwona Romek said he had a wife and child of about 6 whom he would walk to school. He rarely left home in the evening.

“He seemed like a normal family man who liked to take care of his garden,” she said. But one day she saw him packing their belongings in a van and they were gone.

As police investigated, Parliament got back to business, opening the day with a minute of silence for the victims. May saluted the heroism of police and the bravery of ordinary Londoners.

“As I speak, millions will be boarding trains and airplanes to travel to London and to see for themselves the greatest city on Earth,” she said. “It is in these actions — millions of acts of normality — that we find the best response to terrorism. A response that denies our enemies their victory, that refuses to let them win.”

The attack echoed deadly vehicle rampages in Nice, France, and Berlin last year that were claimed by the Islamic State group, also called ISIS.

ISIS said through its Amaq News Agency that the London attacker — whom it did not name — was “a soldier of the Islamic State” who “carried out the operation in response to calls for targeting citizens of the coalition” fighting ISIS in Syria and Iraq.

ISIS has also claimed events later found to have no clear links to it.

Police believe the attacker acted alone, May told lawmakers, with no reason to believe “imminent further attacks” are planned. Britain's threat level from terrorism stands at “severe,” the second-highest on a five-point scale, meaning an attack is highly likely.

Years ago, Khalid was “investigated in relation to concerns about violent extremism,” she said, but called him “a peripheral figure.”