The Finsbury Park terror suspect had been intending to target a Muslim march in London but arrived too late and so allegedly attacked worshippers at a mosque instead, it has been claimed.
Darren Osborne, 47, told locals in a Cardiff pub on Saturday night he was “going to do something about them”, after hearing there was a Ramadan rally planned in the capital the following day.
Hours later police missed an opportunity to prevent the attack when he was reported for being drunk and asleep in the cab of his hire van, but was not arrested.
Mr Osborne, a father of four from Cardiff, remains in custody at a south London police station, where he continues to be questioned in connection with the terrorist attack in north London in which one man died and nine others were injured.
The landlord of his local pub, Andy Parker, claimed the day before the incident he had been ranting about the pro-Palestinian, Al Quds day rally, which took place on Sunday afternoon in central London.
Mr Parker, who runs the Hollybush pub in Pentwyn, said: “The gentleman came in and was very political with everyone he spoke to.
“He was very motivated about the Muslim Al Quds Day rally going on on Sunday and London and kept saying: ‘Our brothers and sisters are dying and someone needs to do something about it’.
“He kept saying he would do something about it, but he kept going on about it, and was saying we need to ‘stand up to Muslims’ it is ‘time we did something about them’.
“I did not like one bit of it so asked him to leave.”
One of the customers added: “He was going on about this march on Sunday and making notes on a piece of paper.
“He just kept saying ‘Al Quds’ and going on about how wrong it was that Muslims were taking over.
“A group of the boys told him to shut up and got into a row with him and eventually he finished his drink and left.
“Now I think about it maybe his plan was to target this march but then went to the mosque when that didn’t work out.”
After leaving the pub, Mr Osborne fell asleep in the white hire van, that was used in the terror attack the following day.
Edward Gardiner, who lives on the same housing estate as Mr Osborne said he tried to wake him up but he was unresponsive so he phoned the police.
Mr Gardiner, 29, said: “He smelled strongly of alcohol and was moaning like you do when you wake up hungover.
“I rang 101 – but I bet they get a lot of calls about drunk people at the wheel so I suppose how would they have known what he was going to go on and do? Hindsight is a beautiful thing isn’t it.”
In a statement a spokesman for South Wales Police said: “At 12.27am on Sunday 18th June South Wales Police responded to a call made to the non-emergency 101 number following a report of an insecure van parked on a street in the Llanedeyrn area of Cardiff.
“Officers attended, 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.”
Exactly 24 hours later it is alleged that Mr Osborne ploughed into a group of worshippers who had just left the Muslim Welfare House mosque, 160 miles away on Seven Sisters Road in north London.
More information about Mr Osborne’s background has emerged, with former associates claiming he had violent tendencies and was barred from every pub in his former town of Weston-super-Mare.
A former landlady in the town said: “There is a pub crime watch here and he was banned from all the pubs in the area, that was 10, maybe 15 years ago.
It is thought he recently split from his partner and had been living in a tent in woodland.
Part of the police investigation will be focusing on whether Mr Osborne had any links to far-Right organisations or extremists.
But there are growing calls for MI5 to play a greater role in investigating such groups, amid claims the growth and sophistication the far-Right has left police too stretched to manage.
Earlier this week Ben Wallace, the Security Minister, warned of that more must be done to tackle far-Right extremism, after Home Office figures showed a steep rise in arrests for domestic terrorism.
At the moment the Security Service focuses on international terrorism, including British based jihadists, and investigations in Northern Ireland.
Domestic terrorism, which is largely dominated by the far-Right, is left to police forces.
But Tom Wilson, a research fellow at the Henry Jackson Society, said: “I think if you see the trends continuing in terms of the ways that the far-Right is communicating and the sorts of ideas being shared, then you would want to bring in the intelligence side to try to undercover these networks.
“You would want to see the same sort of intelligence that’s used to disrupt Islamist extremism.”
Counter terrorism sources said MI5 could play a greater role tackling far-Right terrorism if it became worse, but the service was already stretched dealing with an unprecedented number of Islamist jihadist plots.
A National Police Chiefs’ Council spokesman said: “Tackling the threat from extreme right wing ideology has long been a part of policing’s commitment to fighting extremism in all its forms.
“We are committed to tackling any and all ideologies which pose a threat to the public’s safety and security, and treat the threat from the far right in exactly the same way as any other toxic ideology used to spread mistrust and fear in our communities.
“However, the overriding threat to the UK remains from Daesh-inspired groups and individuals.”