Domingo Tapia and Gary Anderson crossed paths for no more than a second, two lives colliding in a moment of grainy surveillance footage.

Mr. Tapia, a 38-year-old Mexican immigrant who worked as a fruit vendor, had met his brother for a few beers on a summer evening in 2017. They had passed the time and said their goodbyes.

He turned back momentarily to retrieve a bag of fruit he had forgotten at the bar, mounted his bike and glided off toward his wife and two sons, through the quiet streets of Bedford-Stuyvesant, Brooklyn. It was 1:30 a.m.

Two blocks away, Mr. Anderson, a 26-year-old fitness trainer, was standing at Fulton Street and Albany Avenue among a group of men milling around the corner, gesticulating, apparently arguing.

Suddenly, he stalked into the crosswalk, advancing at just the moment Mr. Tapia pedaled into his path. Mr. Anderson took a step, and another, and then he exploded, launching his fist into Mr. Tapia’s face.

Mr. Tapia’s balance failed. The bike spun out. His head smashed against the hard, dark pavement.

The two men didn’t know each other, and they never would. The punch had arrived like many crimes in New York — random, swift, a bolt out of nowhere.