The man suspected of deliberately driving a van into a crowd of worshippers outside a mosque has been identified as Darren Osborne, a 47-year-old father of four.
The vehicle hit pedestrians near Finsbury Park Mosque in north London as they left Ramadan night prayers at around midnight.
A man died and nine other people were taken to hospital following the incident on Seven Sisters Road, which police are treating as a terror attack.
Witnesses described hearing the van driver shout: "I'm going to kill Muslims" and "I've done my bit".
He was restrained by members of the public at the scene, with an imam from the nearby mosque protecting him from angry members of the public.
The alleged attacker has been arrested on suspicion of attempted murder and terror offences.
His relatives say they are "massively shocked" and "their hearts go out to the injured".
His nephew, Ellis Osborne, said the suspect was "not a racist" and said he had never expressed any racist views.
He added: "Its madness. It is obviously sheer madness."
Darren Osborne, who police believe was acting alone, was not known to the security services.
Searches are being carried out at a residential address, where Mr Osborne is listed as living, in the Cardiff area.
Residents in the area have also spoken of their shock.
Taxi driver Saleem Naema, 50, said: "I can't believe it. I know him. I've lived here for five years, he was already living here when I moved in.
"If I ever needed anything he would come. I just can't believe that he did that. I am a Muslim."
Khadijeh Sherizi, who lives next door to Osborne, said: "I saw him on the news and I thought 'oh my God' that is my neighbour.
"He has been so normal. He was in his kitchen yesterday afternoon singing with his kids.
"He was the dad of the family. He has kids. He lives next door. He seemed polite and pleasant to me."
The vehicle used in the attack was rented from Pontyclun Van Hire in South Wales. The firm has said it was "shocked and saddened" at the tragedy.
Police said all the casualties were Muslims.
The man who died had already been receiving first aid from a member of the public, police said.
Prime Minister Theresa May, who chaired a meeting of the COBRA emergency committee, condemned the attack as "every bit as sickening" as the recent atrocities in London and Manchester.
"It is a reminder that terrorism, extremism and hatred take many forms, and our determination to tackle them must be the same whoever is responsible," she said.
Later, she visited Finsbury Park Mosque and met people from a variety of faiths.
Metropolitan Police Commissioner Cressida Dick said it was "quite clearly an attack on Muslims".
There will be more uniformed officers at places of worship, including mosques and Muslim community centres, as police try to reassure local people.