A convicted murderer was among ex-prisoners and members of the public who grappled with and grounded the London Bridge knife attacker before police arrived.

One man was armed with a fire extinguisher and another a 5ft narwhal tusk as people at the scene surrounded the attacker, who was eventually pinned to the ground.

Scotland Yard is investigating how 28-year-old Usman Khan was able to launch the attack in London Bridge, despite being known to the authorities and fitted with an electronic tag to monitor his movements. He was allowed out a year ago after serving time for his part in a plot to blow up the London Stock Exchange.

In footage that has since emerged, Khan is sprayed with a fire extinguisher, while another man tries to suppress the assailant with a narwhal tusk – a long pointed tooth from a type of whale – lunging at him. It is believed the item was pulled from the wall of Fishmongers’ Hall, a Grade II-listed building on London Bridge, by a Polish chef called Łukasz.

Among those who pinned down the attacker was James Ford, 42, who is also thought to have tried to save the life of a woman who had been stabbed. Ford was jailed for life in 2004 for the murder of 21-year-old Amanda Champion.

Ford, who is understood to be serving the final days of his sentence at HMP Standford Hill, an open prison in Kent, was on London Bridge as the attack unfolded.

David Wilson, a professor of criminology at Birmingham City University and chair of the Friends of Grendon prison – where Ford was previously – said the prisoner went through an intensive period of psychotherapy.

“I only picked up it was James Ford as a consequence of them publishing his photo … I remember him and indeed some others from the Friends of Grendon charity.”

He said what happened was a tale of two prisoners, with Ford an example of how people can change. “I know through my work that people do change and they change as a consequence of innovative but challenging regimes such as the one at HMP Grendon.”

Khan was at at Fishmongers’ Hall near London Bridge for a University of Cambridge-organised conference on rehabilitating offenders, after previously participating in the university’s Learning Together prisoner rehabilitation programme, but had showed “no cause for concern”, a source told PA Media.

Wilson said of Khan: “There was a so-called deradicalisation programme and I wonder if Khan went through that … If he went through the deradicalisation programme then it is on another level a tale of two programmes, that programme clearly did not deredicalise him.”

He added: “In the US there is a phenomenon ‘suicide by cop’... increasingly what we are seeing is some attackers wear pretend suicide vests as they prefer martyrdom than getting a sentence. I wonder if what we are seeing is another version of suicide by cop.”

The mayor of London, Sadiq Khan, praised the police and the public for their actions, sharing footage on social media of the people trying to bring the suspect down.

“What’s remarkable about the images we’ve seen is the breathtaking heroism of members of the public who literally ran towards danger, not knowing what confronted them,” he said.

In a tweet, Amy Coop, said: “A guy who was with us at Fishmongers Hall took a 5ft narwhal tusk from the wall and went out to confront the attacker. You can see him standing over the man (with what looks like a white pole) in the video. We were trying to help victims inside but that man’s a hero.”

Craig Heathcote was walking across the bridge as happened. “I was on the east side of the bridge, walking across it – heading to the supermarket on my lunch break. A man appeared in front of me and started screaming that there was someone with a knife, then I realised something was happening,” he told BBC Radio 4’s Today programme.

He said it took a while to process what was going on, but he could see what looked like a “big scuffle happening” and what felt like “hundreds of people screaming” on the bridge.

“The police were not yet there so I called 999 as I felt no one else was [calling them] … While I was on the phone I think a car appeared. It was either the first response or by chance a car driving across the bridge. Someone jumped out and waved them down. Two armed officers jumped out and took over situation,” he said.

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Heathcote said he saw and heard the gunshots as the suspect was shot by police. “It was quite a bizarre moment watching it all unfold,” he said.

Thomas Gray, 24, was among the group who tackled the killer to the ground. He stamped on the terrorist’s wrist to try to make him release one of two large knives he was carrying.

Gray, a tour manager, said: “I was brought up on rugby and the rule is ‘one in, all in’. I did what any Londoner would do and tried to put a stop to it.”

He had been driving northbound on London Bridge with his colleague Stevie when they saw people running. He said they turned the engine off and when they reached the attacker he had been “wrestled” to the floor by “five or six other blokes”.

“He had two knives on him, one in each hand, and it looked like they were taped to his hands,” Gray said.

“I stamped on his left wrist while someone else smacked his hand on the ground and kicked one of the knives away.

The former Metropolitan police Ch Supt Dal Babu said: “The terrorist attack on London Bridge demonstrated the worst and best in society. This cowardly individual sought to kill indiscriminately, but brave Londoners alongside the police prevented further deaths because of their swift actions. They are truly heroes.”