LONDON — A woman’s scream was the first sign John Crilly had of a terrorist attack. For a moment, he wasn’t sure it signaled real distress, he said, but then “it got a lot louder, a lot more intense.”
It sounded like they were both in the same building, the historic Fishmonger’s Hall on the north bank of the River Thames. Rushing to investigate, he saw Saskia Jones, wounded and “sprawled across the stairs with her arms out,” recalled Mr. Crilly, who would be hailed as a hero of the deadly episode on Nov. 29 that ended on London Bridge.
Her attacker, Usman Khan, stood at the bottom of the staircase, a knife in each hand. Mr. Khan said “something like, ‘kill everyone,’ or ‘going to kill you,’ or something about killing people,” Mr. Crilly, 48, told the BBC in an emotional interview published on Thursday.
Risking his own safety, he went at the killer, armed with only the improvised tools at hand — first a wooden lectern, and then a fire extinguisher — as the clash began in a central London building and then spilled out into the street.
He saw that Mr. Khan was wearing what looked like an explosive suicide belt, though it turned out to be a fake.
“I’m just basically screaming at him to blow it,” Mr. Crilly recalled, “like calling his bluff.” He said Mr. Khan, a convicted terrorist, replied that he was waiting to detonate the device until after the police arrived.
“I was prepared to probably lose my life — yeah, I think I was,” Mr. Crilly said.
He and Mr. Khan, 28, both former prison inmates, had gone to a conference held by Learning Together, a prisoner education and reform program created by Cambridge University.
Also there were Ms. Jones, 23, and Mr. Crilly’s friend Jack Merritt, 25, recent Cambridge graduates who were passionate advocates for inmates.
Mr. Khan fatally stabbed Mr. Merritt, a Learning Together supervisor who had worked with Mr. Crilly, and Ms. Jones, a volunteer for the program, and wounded others.
Mr. Crilly and several other people tried to subdue him, and gave chase as the attacker ran from the building and onto London Bridge, scattering the usual weekday throng of pedestrians.
Mr. Crilly sprayed him with the fire extinguisher. Another man poked at the attacker with a narwhal tusk he had pulled from a display in the hall, and another hit him with a pole.
While Mr. Khan slashed at them, they managed to wrestle him to the sidewalk and take away at least one of his knives.
When the police arrived, Mr. Crilly told the BBC, “it seemed like ages before they shot him.”
“I did scream, as well, to shoot him,” he said, “just in case he blew the belt.”
At least one officer fired, killing Mr. Khan.
Mr. Khan had pleaded guilty to plotting terrorist attacks, and served eight years in prison. After claiming to have changed his views, he was released last December.
Mr. Crilly served 13 years. He and a partner broke into a home in Manchester, where they encountered a 71-year-old man who lived there.
According to the BBC, the other burglar punched the resident, killing him. Under Britain’s joint enterprise law, Mr. Crilly was convicted of murder as well as burglary, along with his co-defendant, and sentenced to life in prison.
But Britain’s Supreme Court ruled in 2016 that the law had been interpreted wrongly for years. Mr. Crilly, who completed a law degree in prison, became the only person so far to have a joint enterprise murder conviction overturned under that ruling, and he was released last year.
HMr. Crilly told the BBC that Mr. Merritt had changed his life.
“He actually listened, and you could tell he was really, genuinely interested,” he said. “Jack just basically meant hope.”
Mayor Sadiq Khan of London later cited “the breathtaking heroism of members of the public who literally ran toward danger, not knowing what confronted them.”
But Mr. Crilly said he did not see himself as a hero.
“Jack gave up his life,” he said, closing his teary eyes as his voice broke. “He would be my hero.”