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.