London police have identified the person who carried out Wednesday's attack near the British Parliament as Khalid Masood.
British-born Masood, 52, was known to the police for a number of previous convictions for assaults, including possession of offensive weapons. He was last convicted in 2003 for possession of a knife.
Masood had not, however, been convicted for any terrorism offenses. A religious convert, Masood had been an English teacher and was a fanatical bodybuilder. He is older than recent Islamist attackers in European cities.
Earlier, Prime Minister Theresa May, in comments to Parliament about the attack, said the attacker was once investigated by the country's MI-5 intelligence agency "in relation to concerns about violent extremism." But she said he was not part of what she called the "current intelligence picture," and that there was no prior intelligence on this plot or his motives.
IS claims responsibility
Islamic State has claimed responsibility for the attack which killed three people and ended when an officer shot dead the attacker.
A statement posted on IS's news agency said the attacker was a "soldier of Islamic State." But IS did not indicate that it assisted in the planning or execution of the attack, saying the man had responded to the terror group's call to attack civilians and the military in countries allied with the U.S.-led coalition fighting IS.
One police officer and an American tourist, Kurt Cochran from Utah, have been identified among those killed. The officer was killed on the premises when he was stabbed by the attacker. Cochran succumbed to his injuries in the hospital. His wife was also badly injured in the attack.
In a tweet, President Donald Trump paid tribute to Cochran.
British police have arrested eight people in connection with the attack.
Mark Rowley, head of counterterrorism efforts for London's Metropolitan Police Service, said the arrests came from searches at six addresses, and that "inquiries" at locations in London, Birmingham and other parts of the country were continuing. He did not specify how those arrested were linked to Wednesday's attack.
"It is still our belief, which continues to be born out by our investigation, that this attacker acted alone and was inspired by international terrorism," Rowley told reporters. He added that the attacker's name should not be made public during what he called a "sensitive stage" of the investigation.
The probe right now is focusing on the motives, preparations and associates of the attacker, who ran a car into pedestrians on Westminster Bridge, then stabbed a security officer to death before himself being shot dead by another officer.
Rowley said 29 people have been hospitalized, seven of whom were in critical condition.
Rowley earlier identified the security officer who died in the attack as 48-year-old Keith Palmer, a husband and father who had served in British law enforcement for 15 years.
May strikes defiant tone
May's comments both Wednesday and her remarks to Parliament on Thursday have struck a defiant tone. As Parliament reopened for its normal meetings Thursday, May told lawmakers that what happened Wednesday "was an attack on free people everywhere."
"Yesterday an act of terrorism tried to silence our democracy, but today we meet as normal, as generations have done before us and as future generations will continue to do, to deliver a simple message: We are not afraid and our resolve will never waver in the face of terrorism," she said.
WATCH: May's remarks to parliament
May thanked Britain's friends and allies around the world "who have made it clear that they stand with us at this time." She said the victims include nationals of France, Romania, South Korea, Germany, Poland, Ireland, China, Italy, Greece and the United States.
The United Nations Security Council in New York, currently in meetings chaired by British foreign minister Boris Johnson, held a moment of silence Thursday for the victims of the attack.
Thursday's Parliament in Britain also began with a minute of silence to remember those killed in the attack. Police officers also marked the occasion, standing silently outside the headquarters of London's Metropolitan Police nearby.
"You may know that today there are victims in London from 11 nations. Which goes to show that an attack on London is an attack on the world," Johnson said. "I can tell you from my talks here in the United States with the U.S. government and with partners from around the world that the world is uniting to defeat the people who launched this attack and defeat their bankrupt and odious ideology."
WATCH: Johnson statement at UN
Parliament in Britain also began with a minute of silence Thursday to remember those killed in the attack. Police officers marked the occasion, standing silently outside the headquarters of London's Metropolitan Police nearby.
London is under extra security with a boost in both armed and unarmed officers.
A senior police officer told VOA it appears to have been "a three-staged attack."
It began with an SUV being driven over Westminster Bridge right by the House of Commons. The SUV mounted the sidewalk and struck several pedestrians. According to police sources, the vehicle struck some other pedestrians at the perimeter fence near the gates at Old Palace Yard.
“The attacker then rushed the gates and struggled with a police guard who tried to stop him. The assailant stabbed him several times," the senior police officer said. “Other officers shot the attacker."
In Photos: Britain Parliament Attack Aftermath