Police are appealing for information as they try to establish whether Westminster attacker Khalid Masood acted alone or was directed by others.
They said the investigation would focus on Masood’s “motivation, preparation and his associates”.
Scotland Yard’s top anti-terror officer Mark Rowley revealed that the full birth name of the attacker – who used many aliases – was Adrian Russell Ajao.
It is understood Ajao, who was shot dead, changed his name to Khalid Masood after having a religious conversion.
An old black and white photograph shows Masood as a schoolboy footballer at Huntley Secondary School for Boys in Tunbridge Wells.
His fourth victim has been named as Leslie Rhodes, 75, from Clapham, south London – one of three people killed by Masood’s car on Westminster Bridge.
Police have now released six of the 11 people arrested since Wednesday.
The Metropolitan Police said they had been released “with no further police action” but that four people remained in custody.
A 39-year-old woman, from east London, has been released on bail until late March.
Those still in custody are:
A 32-year-old woman from Manchester
A 35-year-old man from Manchester
A 58-year-old man from Birmingham
A 27-year-old man from Birmingham
Khalid Masood pictured in the school football team
Earlier, in appealing for information from the public, Mark Rowley, of the Metropolitan Police, said they would investigate whether Masood “acted totally alone inspired by terrorist propaganda, or if others have encouraged, supported or directed him”.
The assistant deputy commissioner added: “There might be people out there who did have concerns about Masood but did not feel comfortable for whatever reason in passing those concerns to us.”
Fifty people were injured in Wednesday’s attack, with 31 receiving hospital treatment. Two are in a critical condition, and one has life-threatening injuries..
Masood was described as “very friendly” by the manager of Brighton’s Preston Park Hotel, where he stayed the night before the atrocity.
“He was laughing and joking,” Sabeur Toumi told Sky News, adding that Masood was chatty with staff.
His comments were echoed by guest Michael Petersen, a businessman, who saw Masood in reception on the morning of Wednesday’s rampage.
“The shocking realisation that you can stand next to somebody that is totally, perfectly normal,” Mr Petersen said.
“Articulate, polite, presentable – and the guy is on his way to commit mass murder.”
He launched his attack two minutes later, killing four people. It is unclear whether he was communicating with somebody.
Police have released the first recent picture of Masood, taken within the last year.
Two suspects were detained in Manchester and one in Birmingham.
Fifty people were injured in the London outrage. Two remain critically ill and two police officers have serious injuries.
Islamic State claimed Masood was one of its “soldiers” and “carried out the operation in answer to calls to target the people of coalition states”.
According to a Sky source who met him in a professional capacity, he had a wife and three children.
The source described Masood as “a big bloke”, adding: “He looked like he was a bodybuilder, (you) wouldn’t want to mess with him.”
A businessman tells how he saw terrorist Khalid Masood in the reception of a Brighton hotel.
He added that Masood was a “very religious, well-spoken man”, who had “converted” to Islam.
“You couldn’t go to his home in Birmingham on Friday because he would be at prayer,” the source said.
Masood claimed in a CV to be an English teacher, though it appears he did not work in any of England’s state schools.
Some reports suggested he had spent four years in Saudi Arabia, leading to speculation he may have been radicalised there.
Police said Masood had been “known by a number of aliases”, including Adrian Elms.
While he was born in Kent on Christmas Day 1964, detectives believe he had been living most recently in the West Midlands.
His name is also registered as living at addresses in Brighton, Eastbourne, Luton and London.
A neighbour at his most recent address in Winson Green, Birmingham, said Masood was a keen gardener who left three months ago.
In a statement, the Metropolitan Police said there was “no prior intelligence about his intent to mount a terrorist attack”.
Some people went towards the sound of gunfire when the terror attack in Westminster took place.
But he did have convictions for GBH, possession of offensive weapons and public order offences, spanning 20 years from 1983.
He was jailed for two years in 2000 for slashing a man’s face with a knife in a row with “racial overtones” outside a Sussex pub.
Three years later, after leaving jail, Masood was accused of stabbing a man in the nose, leaving him needing cosmetic surgery.
He was jailed for another six months. He had not been convicted for any terrorism offences, police confirmed.
Earlier, Theresa May said Masood was a “peripheral” figure who had once been investigated by MI5 over concerns about violent extremism.
Anyone with any information about Masood can call the Anti-Terrorist Hotline 0800 789 321.