The suspected Finsbury Park attacker was reported to police for sleeping "drunk" in a van 24 hours before the rampage, according to a witness.
Edward Gardiner, 28, called officers when he saw a man smelling of alcohol asleep in the vehicle with the door open.
He believes it was Darren Osborne, who was arrested after worshippers were hit by a van near the Muslim Welfare House mosque in Finsbury Park, north London, early on Monday.
The vehicle Mr Gardiner saw was rented from Pontyclun Van Hire, near Cardiff, the firm that supplied the one used in the attack.
Mr Gardiner, a self-employed builder and plumber, who lives near Osborne's terraced home in Pentwyn, east Cardiff, called police at 12.27am on Sunday.
"I could smell alcohol on him and he was grunting and groaning. I poked him but he didn't respond so I called 101," he said.
"I thought it was some random drunk guy who wanted to pull over to have a kip. I thought someone should go and check on him.
"I didn't think anything about it but then I saw the news reports and it was the same guy, the same van."
South Wales Police said no offences had been committed when they went to the street in the Llanedeyrn/Pentwyn area of Cardiff.
A spokesman said: "A male was asleep inside the vehicle, which showed no signs of having been driven recently.
"The officers' assessment was that no offences were disclosed."
Labour leader Jeremy Corbyn called for all communities to remain strong against hate and racism as he spoke at a vigil in Finsbury Park on Tuesday.
Osborne, 47, has been arrested on suspicion of commission, preparation or instigation of terrorism, including murder and attempted murder.
The father-of-four was described by relatives as having been "troubled for a long time".
In a statement his family said: "We are massively shocked. It's unbelievable. It still hasn't really sunk in.
"We are devastated for the families. Our hearts go out to the people who have been injured."
They said Osborne was "not a racist" and had never expressed racist views, adding: "It's madness. It is obviously sheer madness."
The attack unfolded as a man who had been taken ill was receiving first aid from the public near the mosque, where people had been saying Ramadan night prayers. The van was driven at people who were helping him.
The man was pronounced dead at the scene and paramedics took nine other people to three London hospitals while two others were treated for minor injuries.
Witnesses described hearing the driver, who was detained by members of the public before police arrived, shout: "I am going to kill Muslims."
Osborne had been ejected from the Hollybush pub on Saturday night after arguing about immigration.
Pub regular Phil Henry said: "He was kicked out of here, the landlord said, for shouting about Islam.
"When he came in the landlord said he was on his own and started shouting, 'I'll kill every f****** Muslim'.
"They said, 'We don't want to hear that', and he just left of his own accord."
Neighbours said Osborne had been living in a tent in woodland after being kicked out of the family home.
Khadijeh Sherizi, who converted to Islam in 2003, said he was polite and her children, who are Muslim, would play with his children most days.
But on Saturday, she said, he approached her eldest son, 12, and called him "inbred" and repeated the insult to her 10-year-old daughter the next day.
"We are Muslim but I am white," Mrs Sherizi said.
"He was just so normal. We never had a problem with him, nothing."
Detectives have been granted a warrant to hold Osborne until 12.54am on Saturday.
Chief Superintendent Dave Stringer, head of community engagement at the Metropolitan Police, said additional officers will be patrolling across the city and at places of worship.
Deputy Assistant Commissioner Lucy D'Orsi, the National Police Chiefs' Council lead for protective security, said: "Police forces up and down the country are being asked to review their security measures around places of worship, including mosques and Muslim community centres.
"But we can all play a part in keeping our friends, families and fellow citizens safe from terror, and that is by being vigilant to anything unusual, and raising any concerns about suspicious activity or behaviour to police."