Khalid Masood, a 52-year-old Briton with a history of violent offences but no terrorist convictions, was shot by police on Wednesday after a rampage through Westminster.
Four other suspects are in custody following searches at 21 locations, mostly around London, in the central city of Birmingham and in the northwestern city of Manchester.
The Islamic State group claimed that Britain's deadliest terror attack in 12 years was conducted by one of its "soldiers" acting on a call to target countries in the US-led coalition fighting the jihadists.
Police said there was no further threat, but counter-terrorism commander Mark Rowley said they were trying to establish whether Masood acted alone "or if others have encouraged, supported or directed him".
Released by police, the passport-style photo of Masood, who also used the names Adrian Elms and Adrian Russell Ajao among other aliases, shows a black man with a shaved head and a beard.
Prime Minister Theresa May has said he was known to intelligence services as a "peripheral" figure some years ago but there was no warning of his plans to stage an attack.
At least 50 people from 12 different countries were injured when Masood ploughed his rented car into crowds of people walking along the pavement on Westminster Bridge, before crashing the vehicle into the fence outside parliament.
Three people on the bridge died after being hit by the speeding car then the attacker leapt out and fatally stabbed a police officer just inside the gates of the Houses of Parliament before being shot dead.
Rowley described the overnight arrests of two men aged 27 and 35 in Birmingham and Manchester as "significant"
Numerous aliases
Prince Charles, the heir to the British throne, visited some of the victims in hospital on Friday, thanking staff for their "marvellous efforts".
The death toll rose late Thursday after life support was withdrawn from a 75-year-old man injured in the attack, whom police named as Leslie Rhodes from south London.
The other victims were 48-year-old policeman Keith Palmer and, on the bridge, 43-year-old Briton Aysha Frade, who was on her way to pick up her two daughters, and an American citizen in his 50s, Kurt Cochran.
Born in Kent in southeast England, Masood was a British citizen with convictions for assault and possession of offensive weapons dating from 1983 to 2003.
A report dating back to November 2000 shows he was sentenced to two years in prison for slashing the face of a cafe owner in a village in southern England where he lived with his family.
According to The Sun tabloid, he married a Muslim woman in 2004 and moved the following year to Saudi Arabia to teach English, returning in 2009, but officials could not confirm this.
The Saudi embassy in London on Friday issued a statement saying that Masood was not known to their security services "and does not have a criminal record in the Kingdom of Saudi Arabia."
Police chief Rowley said police were "looking at his history" and appealed to the public to come forward with any information about him.
"Our investigation focuses on understanding his motivation, preparation and associates," he said.
A total of 11 people have been arrested over the attack, their ages ranging from 21 to 58, all on suspicion of preparation of terrorist attacks.
Three of the women held have since been released, one on bail, while four of the men were also released, according to police.
Peaceful future
Faith leaders including Archbishop of Canterbury Justin Welby, the chief rabbi and the chief imam of London's central mosque on Friday gathered near the attack site to remember the victims.
"In standing here, we represent the three Abrahamic faith communities, all equally committed to a peaceful future," Welby said after observing a minute's silence outside Westminster Abbey.
Lawmakers returned to work as normal on Thursday morning, even as forensic officers worked at the scene, but a review of parliamentary security is now under way.
Conservative MP Tobias Ellwood, whose attempts to save police officer Palmer were widely praised, was on Friday appointed to the Privy Council, a prestigious role usually reserved for senior ministers, as a reward for his efforts.
The IS group said it was responsible for Masood's actions without naming him, according to the Amaq propaganda agency -- its first claim for an attack on British soil.
It was the deadliest in Britain since four homegrown suicide bombers killed 52 people on the city's transport system in July 2005.
The attack had echoes of the atrocities in Nice and Berlin when trucks ploughed into crowds of people, killing 86 people in the French Riviera city in July and 12 at a market in the German capital just days before Christmas.
