Update 2 | Eight arrested after Westminster terrorist attack leaves four dead

Eight arrests have been made following raids after an attack on Westminster left four dead yesterday, according to police.

Police and security services have been investigating the background of the man who killed three people and injured dozens in a terrorist rampage at Westminster on Wednesday, before being shot dead.

Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley said hundreds of detectives have worked through the night, carrying out searches at six addresses in London and Birmingham, arresting eight people.

BBC Newsnight reported that the vehicle used in the attack – a Hyundai i40 – could have been rented in Birmingham. Enterprise car rental company confirmed the car "was one of ours", adding that it alerted police as soon as they realised the car belonged to them.

"An employee identified the vehicle after seeing the licence plate in an image online. We ran another check to verify, and immediately contacted the authorities. We are co-operating fully with the authorities and will provide any assistance that we can to the investigation," a spokesperson for Enterprise Holdings said.

Officers were attempting to establish the motivation and any terrorist links of the attacker and look into his connections and associates. According to UK Prime Minister Theresa May, he was British-born, and investigated some years ago in relation to extremist terrorism, but was a “peripheral figure”.

UK PM Theresa May clarifies the latest on the investigation into the #WestminsterAttack More: https://t.co/0k9Myetb0n pic.twitter.com/7vq1adgvPk

— Reuters Top News (@Reuters) March 23, 2017

“The case is historic. He is not part of the current intelligence picture,” she added.

The attacker, who has not been named by police, ploughed through pedestrians on Westminster Bridge before ramming into the gates of the UK parliament and stabbing 48-year-old unarmed police officer and father Keith Palmer. A member of the Parliamentary and Diplomatic Protection Service and a police officer for 15 years, Palmer later died from his injuries.

Keith Palmer, a husband, father and police officer for 15 years, was killed in the attack

The Met’s head of counter-terrorism, Mark Rowley revised the death toll down to four people, including the assailant and Palmer.

The mayor of the Spanish town of Betanzos, Ramón García Vázquez, confirmed one of the deceased to be 43-year-old Aysha Frade, a Spanish teacher and mother of two. US tourist Kurt Cochran was later named as the third victim.

His wife Melissa has been seriously injured and is in hospital, along with 12 Britons, three French, two Romanians, four South Koreans, one German, one Chinese, one Irish, one Italian, one Pole and two Greeks. Seven of the injured are in a critical condition.

WATCH: British PM Theresa May condemns today's #Westminster attack as 'sick and depraved.' https://t.co/yPKLN1scu6 pic.twitter.com/kPPQEuNKg7

— Reuters Top News (@Reuters) March 22, 2017

May said the attack was "sick and depraved" and struck at values of liberty, democracy and freedom of speech.

The Prime Minister added: "The location of this attack was no accident.

"The terrorist chose to strike at the heart of our capital city where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech."

In her address, May affirmed parliament would meet as normal on Thursday.

“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 waiver in the face of terrorism.”

A man believed to be a suspect in the attack pictured being treated by paramedics

A 4x4 car rammed into Westminster gate

A man being assisted after being thrown down a staircase during the Westminster attack

Injured people are assisted after an incident on Westminster Bridge in London

She added: “The greatest response lies not in the words of politicians, but in the everyday actions of ordinary people. For beyond these walls today – in scenes repeated in towns and cities across the country –millions of people are going about their days and getting on with their lives. The streets are as busy as ever. The offices full. The coffee shops and cafes bustling. As I speak millions will be boarding trains and aeroplanes to travel to London, and to see for themselves the greatest city on Earth.

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. That shows we will never give in. A response driven by that same spirit that drove a husband and father to put himself between us and our attacker, and to pay the ultimate price.”

Rowley said the motivation of the attacker was assumed to be “Islamist related” and he had tried to enter parliament but had been stopped.

Theresa May commends the actions of ordinary people: https://t.co/hO1zNr2xgD pic.twitter.com/XriuNqjcBx

— Reuters UK (@ReutersUK) March 23, 2017

"The inquiries in Birmingham, London and other parts of the country are continuing. It is still our belief - which continues to be borne out by our investigation - that this attacker acted alone and was inspired by international terrorism," he said. "To be explicit, at this stage we have no specific information about further threats to the public."

On Thursday afternoon, ISIS claimed responsibility for the attack, making the announcement on the group’s Amaq news agency.

"The perpetrator of the attacks yesterday in front of the British parliament in London is an Islamic State soldier and he carried out the operation in response to calls to target citizens of the coalition," the Amaq statement said.

In London, extra officers – armed and unarmed – have been ordered onto the streets and Parliament Square remains closed as the capital wakes up. Westminster Bridge remains closed and the tube station is open but for interchange only.

London mayor Sadiq Khan advised Londoners that additional armed and unarmed police officers on patrol. He reassured to those in the city not to be alarmed and that London remains safe. “We stand together in the face of those who seek to harm us and destroy our way of life. We always have and we always will. Londoners will never be cowed by terrorism."

Khan has issued an open invitation to all Londoners and visitors to the capital to a candle-lit vigil in Trafalgar Square from 7:00pm this evening.

"The Mayor is asking everyone to come together in solidarity to remember those who have lost their lives, to express sympathy with their families and loved ones and to show the world that we are more committed than ever to the values that we hold dear – that we remain united and open," a City Hall statement read.

The attack occurred on the one-year anniversary of the Brussels attacks which killed 32 people.