Khalid Masood, 52, has been identified as the man behind the deadly attacks Wednesday in London outside Britain's Parliament.

The death toll from the attack rose to four on Thursday when police announced a 75-year-old man "died tonight after his life-support was withdrawn."

Masood was born in Kent to the southeast of London and had been most recently living in central England, London police said.

"Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack," police said in a statement. "However, he was known to police and has a range of previous convictions for assaults, including GBH [grievous bodily harm], possession of offensive weapons and public order offences."

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People lit candles Thursday night at a vigil in Trafalgar Square in London the day after the attack. (Hannah McKay/Reuters)

Several thousand people gathered Thursday evening in London's Trafalgar Square for a candlelit vigil to honour the victims and their families and to show London's unity in the face of militant attacks.

Dignitaries, police and clergy, including Buddhist monks in brown robes, gathered on the steps leading to the National Gallery. There was a strong sense of solidarity and camaraderie, with Muslims mingling with Jews and people stopping to thank the police for their work. After speeches and a minute's silence Amber Rudd, the home secretary MP, mayor Sadiq Khan and acting police commissioner Craig Mackey lit the three oversize candles to applause from the crowd

Earlier, U.K. Prime Minister Theresa May said the attacker had been investigated by British intelligence "some years ago."

May said he had been investigated by MI-5, the U.K.'s intelligence service, "in relation to concerns of violent extremism," but that he was not part of what she called the current intelligence picture, saying he was a "peripheral figure."

'Our values will prevail': British PM10:47

Masood sped across Westminster Bridge in a car, ploughing into pedestrians along the way, then ran through the gates of the nearby Parliament building and fatally stabbed an unarmed policeman before being shot dead.

May and British police say they believe he acted alone and was "inspired by international terrorism." She said there is no reason to believe that "imminent further attacks" are planned.

She called Wednesday's events "an attack on free people everywhere."

ISIS, the Islamic State in Iraq and Syria, claimed responsibility for the attack, the group's Amaq news agency said on Thursday. The group has been responsible for numerous bloody attacks around the globe, but also has claimed certain attacks in a show of opportunism.

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Thousands gathered during a candlelit vigil at Trafalgar Square in London on March 23, where a moment of silence was observed. (Jack Taylor/Getty Images)

Forty people were injured and 29 remain in hospital, seven of whom are in critical condition. 

Those still in hospital included Britons and people from 10 other countries, including an American and three children from France.

Victims killed and injured

A Mormon church official says a Utah man was one of the people killed and his wife was seriously wounded and remains in hospital. Kurt W. Cochran and his wife, Melissa, were on the last day of a trip celebrating their 25th wedding anniversary on Wednesday when the attacker struck.

The church said Thursday the Utah couple was also visiting the woman's parents who are Mormon missionaries in London.

And earlier, the Spanish Foreign Ministry said a British national whose mother is Spanish has been identified as one of the people killed.

Aysha Frade

One of the victims the March 22, 2017, London terror attack has been named as 43-year-old Aysha Frade. (BBC)

A spokesman said that Spanish consular officials made contact with Aysha Frade's relatives in Spain's northwestern region of Galicia after her identity was confirmed. He was unable to give any further details.

The regional Voz de Galicia newspaper said that Frade was 43 years old and was teaching Spanish in London.

The identity of the 75-year-old who died Thursday was not immediately clear.

Queen sends condolences

Queen Elizabeth sent her condolences on Thursday to those affected by the attack.

"My thoughts, prayers, and deepest sympathy are with all those who have been affected by yesterday's awful violence," she said in a statement. The Queen was due to officially open the new headquarters of London's police force on Thursday but will now do so at a later date.

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Police and forensic officers work in London on Thursday. (Hannah McKay/Reuters)

Since the attack, police have raided at least six addresses, including in London and Birmingham, which have resulted in eight arrests in total.

Mark Rowley, assistant commissioner with the Metropolitan Police, said they were continuing to investigate the attacker's "motivation, his preparation and his associates."

May and Rowley said there will be an increased police presence in the city, including armed and unarmed officers, as well as more patrols.

MP tried to save officer's life

The policeman killed was identified late Wednesday as Keith Palmer, 48.

Conservative MP Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed first aid on the wounded police officer, who later died. About 10 yards away lay the assailant.

"I tried to stem the flow of blood and give mouth to mouth while waiting for the medics to arrive but I think he had lost too much blood," Ellwood said. "He had multiple wounds, under the arm and in the back."

May praised Palmer, calling him a hero who "devoted his life to the service of his country." She said the husband and father had been a member of the parliamentary and diplomatic protection command for 15 years. He had also been a soldier in the Royal Artillery. 

In Parliament Thursday, Conservative MP James Cleverly called for Palmer to receive posthumous recognition for his "gallantry and sacrifice."

Choking back tears, Cleverly said he first met Palmer 25 years ago as "Gunner Keith Palmer" when both men served in the Royal Artillery.

Cleverly said Palmer was "a strong, professional public servant and it was a delight to meet him again only a few months after being elected."

Here is how the attack on Wednesday unfolded: 

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With files from CBC News and Reuters