After police raided a housing complex in Barking, east London, and made several arrests, residents recognized a familiar face among the three dead attackers and one said she had reported his behavior to police.
Barking resident Erica Gasperri said she went to the police after she saw a man, believed to be the attacker, teaching the local children about Islam.
"All of a sudden we saw this individual speaking to the kids ... showing them how to pray. He was standing over there, I could see them from my window," Gasperri told CNN's Melissa Bell.
Gasperri said that she confronted the man and complained that little was done after she reported him to the police.
Residents at the complex said that the man in question was quiet and kept to himself.
People in the area said the man had joined a mosque nearby after he had an argument with the community at another and abruptly left.
British authorities here have so far refused either to confirm or deny the man's identity, but have said they have identified all three attackers and will name them in due course.
The BBC's Asian Network interviewed a man who said he knew one of the attackers and had reported him to the police. It was not clear if he was speaking about the same man Gasperri knew.
"We spoke about a particular attack that happened and, like most radicals, he had a justification for anything - everything and anything.
"And that day I realized that I need to contact the authorities," he said, without giving his name.
He said that the police had not taken action.
"I did my bit ... but the authorities didn't do their bit."
Police 'completely overwhelmed'
The accusations were made as Prime Minister Theresa May came under fire
for slashing some 20,000 police posts across the country is a money-saving exercise.
May was Home Secretary and oversaw security at the time the Conservative government made the cuts to the police force.
The police have been praised for their quick response to Saturday's attack, storming bars and restaurants in Borough Market to keep members of the public safe.
But Britain's most senior police officer, Cressida Dick, said that as the threat to the country's security changes, police resourcing and strategy should be revised.
The Soufan Group, a security intelligence consultancy, said that security services in the UK were facing "a threat matrix that, like in other European countries such as France and Germany, completely overwhelms their ability to effectively assess and prioritize threats."
"With as many as 3,000 or more individuals considered serious threats -- and thousands more at various other levels of concern -- the UK's intelligence and security organizations are struggling to keep up with a problem that shows no sign of abating."
The Soufan Group's Stephen White, who worked as a senior UK police officer for 26 years, explained that weighing up which reports to respond to was a complex task for the police.
"It does look like tips should have been followed, but if they have to respond every time a member of the community waves a black flag or expresses radical view, we have to ask, do they have the resources to put everyone under surveillance?"
"The Issue here is that there seems to have been more than one piece of information reported to police, so we will need to wait and see what happened with that information. Was it not acted on? Was there a proportionate judgement? Was it not taken seriously enough? It is complex, and there are a lot of questions to answer."