This as new details are emerging about the alleged mastermind of the attack.
Authorities on Thursday identified a 52-year-old Briton as the man who mowed down pedestrians and stabbed an officer to death outside Parliament in London, 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."
RELATED: 7 arrests made in raids following London terror attacks
"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.
A 75-year-old victim on the bridge died late 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.
Timeline of events: The London terror attacks
"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 earlier 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.
Man, 75, dies of wounds in London terror attack
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. Shown a photo of him, Iwona Romek said "that is 100 percent" the man who lived next door to her for about five months.
Romek said he had a wife and child about 6 years old who he would walk to school. He rarely left home in the evening.
Houstonians from Great Britain react to attacks
"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 black van and they were gone.
As police investigated, Parliament got back to business, opening the day with a minute's 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."
In 1,000-year-old Westminster Hall, the oldest part of Parliament's buildings, politicians, journalists and parliamentary staff lined up to sign a book of condolences. One uniformed policeman wrote: "Keith, my friend, will miss you."
Some parliamentarians said they were shaken by Wednesday's attack, and all were somber. But they also were determined.
"There is no such thing as 100 percent security," said Menzies Campbell, a member of the House of Lords. "We have to learn to live with that."
The attack echoed deadly vehicle rampages in Nice, France, and Berlin last year that were claimed by the Islamic State group.
IS said through its Aamaq 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 IS in Syria and Iraq.
IS has been responsible for violence around the globe although the group 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."
Home Secretary Amber Rudd denied there had been an intelligence failure because the attacker had been known to police.
"I think that would be absolutely the wrong judgment to make," Rudd told the BBC. "I'm confident that as we get more information ... that we will learn more and take comfort from the information that we have and the work that the intelligence services do."
British security forces say they have foiled 13 plots in the past four years.
London has been a terrorism target many times. Last weekend, hundreds of police simulated a attack on a tourist boat on the River Thames, which winds through London.
The victims were from 11 countries. They included 12 Britons, four South Koreans, three French, two Romanians, two Greeks, two Irish, two Americans and one person each from Germany, Poland, China and Italy.
Cochran, who was visiting London with his wife, Melissa, for their 25th anniversary, was listed among the dead by the Church of Jesus Christ of Latter-day Saints. His wife was seriously injured and hospitalized.
Nigel Farage, former leader of the right-wing U.K. Independence Party, blamed the attack on "multiculturalism."
But most politicians said the violence should not divide Britain, and May called the rampage "a perversion of a great faith."
As dusk fell, a silent vigil was held by several thousand people in London's Trafalgar Square, where the bells of Big Ben could be heard in the distance.
"Those evil and tortured individuals who try to destroy our shared way of life will never succeed," Mayor Sadiq Khan told the crowd.
Sughra Ahmed, a Muslim woman who traveled from northwest England for the vigil, said she'd been reduced to tears on the square by a woman who embraced her.
"Britain is one," she said. "An attack on one is an attack on us all."
Report a typo to the ABC13 digital staff