The man who attacked London leaving three dead was a British national with known links to religious extremism, said the British Prime Minister on Thursday.

In a speech to the lower house of Parliament, Theresa May said the attacker was believed to have acted alone and had some years ago been investigated by the interior security service MI5.

The Islamic State terror organization claimed authorship of the attack in an unverfiable statement realeased on the group's online portal.

On Wednesday, the assailant drove a car through crowds of pedestrians on Westminster Bridge killing two: Aysha Frade, a 43-year-old British national of Spanish origin, and an unnamed man in his fifties. He then attacked police with a knife, killing Police Constable Keith Palmer before being shot dead by armed officers outside Parliament.

Around 29 people were injured in the attack, of which seven are in critical condition, including a Romanian woman who fell into the River Thames.

"This was an attack on free people everywhere and on behalf of the British people I would like to thank our friends and allies around the world who have made it clear they stand with us," said May.

Counter-terror raids across London and Birmingham led to at least eight people being arrested on suspicion of having links to the attack, announced the London Metropolitan Police earlier Thursday.

The assailant appeared to have acted alone, inspired by international terrorism, the police said.

On Wednesday, Prime Minister Theresa May denounced “the sick and depraved terrorist attack,” in which the assailant mowed down pedestrians in a sport utility vehicle, crashing outside Parliament. He then fatally stabbed a police officer before being shot and killed.

The investigation into the terrorist attack outside the Houses of Parliament in London is continuing. At least five people have died, including a policeman and the attacker.

The attack just a year and a day after 32 people were killed and more than 320 injured in coordinated attacks in Brussels. 

World leaders condemned the attack and offered condolences. US president Donald Trump spoke to May, promising the UK the full support of the US government in responding to the attack.

Rex Tillerson, the US secretary of state, said the thoughts and prayers of the American people were with the UK after Wednesday’s atrocities. “We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference,” he said.

Here’s a look at what we know about the attack: .

- A teacher and mother of two was among those who died in Wednesday’s terror attack in London. Aysha Frade, 43, who worked as a teacher in London, had family in the Spanish town of Betanzos, Galicia, northwest Spain.

- Approximately 40 other people were injured. Several are still being treated for serious injuries, including two police officers, as reported in The Guardian.

-The assailant drove a car into pedestrians on Westminster Bridge, killing two people, before crashing it outside parliament and trying to enter the complex, armed with a knife.

He stabbed an unarmed police officer who later died from the injuries, before armed police shot him. 

Investigators say they believe they know the identity of the attacker, and that he is thought to have acted alone but was “inspired by international terrorism”. They are investigating possible associates.

- Three French high school students, four British university students and two Romanians were among those injured when the attacker drove into pedestrians.

- Five South Korean tourists and a German woman resident in Australia have also been identified as among the wounded.

- One woman was thrown or jumped into the Thames from the bridge. She was rescued from the water but had sustained serious injuries.

Follow live coverage of the London attack investigations in The Guardian.