The man responsible for the terrorist attack in Westminster has been named by police as Khalid Masood.
Masood, 52, was born in Kent and detectives believe he was most recently living in the West Midlands.
Neighbours of Masood said they were shocked to learn of his involvement of the attack, saying he was a "very calm" person.
He is thought to have had three children — two girls of primary school age and one boy of around three.
Scotland Yard said Masood had previous convictions for assaults, including GBH, possession of offensive weapons and public order offences.
His first conviction was in November 1983 for criminal damage and his last conviction was in December 2003 for possession of a knife.
Although he had not been convicted of any terrorism offences, security sources said Masood had associated in the past with suspected Islamist terrorists but had but had not been part of a plot himself.
His indoctrination, it is believed, may have begun through meeting extremists while in prison.
Masood was also known by a number of aliases, the Metropolitan Police said.
Three people were killed when the knife-wielding attacker ploughed a car through pedestrians on Westminster Bridge, before storming the Parliamentary estate. He was shot dead.
Masood had been living at a property in Birmingham's Winson Green neighbourhood until December, according to The Birmingham Mail.
The paper reported that he had been living there with a woman and a young child, until December when he left the property. He is also thought to have worked as an English teacher and held an interest in bodybuilding.
Iwona Romek, a factory worker who claimed to be Masood's neighbour, said the killer had a "nice family".
"They were a nice family, very reserved," she told The Guardian. "He was very calm I saw the photos on the TV and knew it was the man who lived here. He had a wife, a young Asian woman, and a small child who went to school."
Earlier, Prime Minister Theresa May said the attacker was born in Britain and had previously been investigated for suspected extremism by MI5 .
He was once investigated in relation to concerns about violent extremism some years ago but was a "peripheral figure".
The case is "historic" and the attacker was "not part of the current intelligence picture," Ms May added.
Isis claimed the attacker was a "soldier of the Islamic State" when it claimed responsibility for the attack.
However the group did not name him and did not provide any further details. The terrorist group regularly claims “credit” for atrocities and there is no evidence at present to verify the claim. No video has yet surfaced of him pledging adherence to Isis and its leader Abu Bakr al-Baghdadi, which has become the practice of those carrying out attacks on behalf of Isis.
Police arrested eight people at six locations in London and Birmingham as part of its investigation into the attack, which Ms May said was inspired by a warped Islamist ideology.
The Enterprise rental car company said the vehicle used in the attack had been rented from its Spring Hill branch in Birmingham, which is located in the West Midlands.
"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," said company spokesman John Davies.
About 40 people were injured in the attack, of whom around 29 remain in hospital.
Five people remain in a critical condition and two have life threatening injuries.
Police had given the death toll as five but revised it to four.
The casualties included 12 Britons, three French children, two Romanians, four South Koreans, one German, one Pole, one Chinese, one American and two Greeks, Ms May said.
The attack took place on the first anniversary of attacks that killed 32 people in Brussels, and resembled Isis-inspired attacks in France and Germany where vehicles were driven into crowds.
It was the deadliest attack in Britain since 2005, when 52 people were killed by Islamist suicide bombers on London's public transport system.
Police asked anyone with information about Masood to contact the Anti-Terrorist Hotline on 0800 789 321
