Just hours before Pakistani-born British man Khuram Shazad Butt and two accomplices drove a rented van into pedestrians on London Bridge and stabbed people nearby, he was asking neighbours where he could hire a vehicle.

One of the neighbours, Ikenna Chigbo, said he recognised Butt from a photo issued by British police following Saturday night's attack in the capital which killed seven and injured nearly 50.

Chigbo told Reuters that the man he knew as 'Abz' had shown interest in a van he had rented.

"I was in the process of moving house, he was looking at the van, asking me where I hired it, how much it cost, and if the vans were available in automatic," said Chigbo, who lived in a neighbouring apartment in Barking, east London.

"That was Saturday, approximately 3pm. He came across... almost euphoric. He just seemed on a whole different level. It was quite strange."

Reuters could not independently verify the identity of the man who spoke to Chigbo.

However, a neighbour quoted by The Guardian said he saw Butt with the van on Friday.

Michael Mimbo, 25, who lived in a nearby block of flats said he had known Butt for three years.

"I saw him with the van on Friday... He had parked it right outside the flats and it was blocking the road. Cars couldn’t get past and there was a bit of aggro," he said, adding that several neighbours had seen the van being driven the wrong way up a one-way street outside the flats.

Shortly before 10pm on Saturday night, Butt — along with Rachid Redouane and another man not yet named by police — began their rampage across London Bridge and through the bustling Borough Market area. All three were shot dead at the scene by police.

Butt, 27, a keen fan of Arsenal soccer club, was known to the intelligence services but not thought to be actively plotting an attack.

His conversation with Chigbo also reinforces suggestions that Britain's third deadly militant incident in just three months may have involved little planning or sophistication.

Police have said they need a major re-think about how to counter such plots by people who had come onto their radar but were not considered a serious threat.

"We are all needing to look very hard at our strategy at what we've been doing," said Cressida Dick, London's police chief. "In my view we will need to change again in the future."

Saturday's attack follows a similar method first seen in Britain four years ago when two British Muslim converts used a car to mow down soldier Lee Rigby before stabbing him to death.

The London Bridge rampage was almost a direct copy of the crude attack in March when Khalid Masood ploughed into pedestrians on London's Westminster Bridge before stabbing a policeman to death in the grounds of parliament.

Neither attack seemed to be the result of any long-term planning nor, despite the claim of responsibility by the militant Islamic State (IS), any serious foreign direction.

"We are dealing with people who appear very volatile, very unstable many of them, people who are prepared to use low-tech methods, (who) sometimes go from thinking about the idea to carrying out the attack in a very short space of time," Dick said, adding further copycat attacks were a possibility.

'Terrorism breeds terrorism'

British Prime Minister Theresa May has said Britain is faced with a new trend where “terrorism breeds terrorism” and inspired not just by “carefully-constructed plots after years of planning and training and not even as lone attackers radicalised online but by copying one another and often using the crudest of means of attack”.

Britain's domestic intelligence agency MI5 has identified about 23,000 people as potentially violent militants with 3,000 of these considered to pose a threat. It is currently running 500 active investigations.

“It is a very large number. It's very, very hard. You only have to think how would you go about keeping under very active surveillance over 20,000 people,” Dick said.

Most of those involved in the three recent attacks were known to the security services, but none had been considered to pose a threat that required extra surveillance.

The Westminster Bridge attacker had shown up on the periphery of previous terrorism investigations that brought Masood to MI5's notice, but not enough to warrant further action. Investigators now believe he was a “lone wolf”, who radicalised himself through material on the internet.

Salman Abedi — the 22-year-old born in Britain to Libyan parents who killed 22 people in a suicide attack on a pop concert in Manchester last month — was also known to MI5 but not under active investigation.

The BBC reported concerns had been raised with the authorities on at least three occasions.

Butt was known to police and MI5. He appeared in a TV documentary last year about a radical group which was seen at one stage unfurling an IS flag.

One of his friends told BBC Asian Network he had phoned an anti-terrorist hotline set up by British authorities because he feared Butt had been radicalised after watching clips of US preacher Ahmad Musa Jebril, whose online sermons have been a leading source of inspiration for foreign fighters in Syria.

“I did my bit, I know a lot of people did but the authorities didn't do their bit and that's what's shocking,” the friend, who the BBC did not identify, said.

Another neighbour also said she had reported Butt to police two years ago after she feared he was radicalising children in a local park.

“There was no intelligence to suggest that this attack was being planned and the investigation had been prioritised accordingly,” police said in a statement.

A report by Britain's Intelligence and Security Committee into Rigby's murder in May 2013 concluded the security services could not have stopped the attack.

However, one of the killers had been investigated five times, twice as a high priority, and it said more needed to be done in dealing with low-level suspects.

With finite resources, the authorities have to weigh up who poses the greatest threat and needs intensive monitoring.

At least two dozen people are needed to carry out 24-hour surveillance on just one suspect and about 50 officers can be required at times.

“It's costly and difficult and it's hard to know what's going on in people's minds,” Commissioner Dick said.

The authorities say they have foiled 18 plots since Rigby's killing, including five since the Westminster Bridge attack in March.

Raffaello Pantucci, Director of International Security Studies at the Royal United Services Institute think-tank, said it was not clear whether intelligence agencies would be able to develop a system which could cope with such low-tech plots.

“You can do all the sort of calculations you want ... but ultimately it's going to be difficult if all it takes to do a plot is to get a knife and drive your car into people,” he said.

“In a way it would be even worse if they didn't know who these people were, and were looking in completely the wrong place.”