The man arrested on suspicion of carrying out a terror attack near a north London mosque is Darren Osborne from Cardiff, the BBC understands.
The father-of-four was held after a van hit Muslims who had been attending evening prayers in Finsbury Park.
They had been helping a man who had collapsed. He later died but it is not clear if it was because of the attack.
Osborne, 47, was held on suspicion of attempted murder and later further arrested over alleged terror offences.
Police are carrying out searches at an address in the Cardiff area.
Security Minister Ben Wallace said the suspect was not known to the security services, and was believed to have acted alone.
The BBC understands Osborne grew up in Weston-super-Mare, Somerset.
Osborne's family members gathered outside a house in the town, with his sister Nicola telling the Press Association: "I am very sorry for what's happened."
The Metropolitan Police said he was being held on suspicion of the commission, preparation or instigation of terrorism including murder and attempted murder.
The attack took place shortly after midnight close to Muslim Welfare House on Seven Sisters Road, which is also a community centre.
Several of the injured are believed to be seriously hurt.
London Ambulance Service said nine people were taken to hospital and a number of others were treated at the scene for minor injuries.
Met Police Commissioner Cressida Dick said the incident was "quite clearly an attack on Muslims", and the community would now see more police, including armed officers, in the area, "particularly around religious establishments".
It is the fourth terror attack in the UK in three months, after incidents in Westminster, Manchester and on London Bridge.
Prime Minister Theresa May said the attack is "every bit as sickening" as the others.
"It was an attack that once again targeted the ordinary and the innocent going about their daily lives - this time British Muslims as they left a mosque having broken their fast and prayed together at this sacred time of year," she said.
After speaking outside Downing Street, the prime minister visited Finsbury Park Mosque, which is also close to the scene of the incident, where she held talks with faith leaders.
Labour leader and Islington North MP Jeremy Corbyn also visited the area, telling the BBC that "an attack on a mosque, an attack on a synagogue, an attack on a church is actually an attack on all of us".