Two police officers prepared to enter the pitch-black eighth-floor stairwell of a building in a Brooklyn housing project, one of them with his sidearm drawn. At the same time, a man and his girlfriend, frustrated by a long wait for an elevator, entered the seventh-floor stairwell, 14 steps below. In the darkness, a shot rang out from the officer’s gun, and the 28-year-old man below was struck in the chest and, soon after, fell dead.

The shooting, at 11:15 p.m. on Thursday, invited immediate comparison to the fatal shooting of an unarmed man in Ferguson, Mo. But 12 hours later, just after noon on Friday, the New York police commissioner, William J. Bratton, announced that the shooting was accidental and that the victim, Akai Gurley, had done nothing to provoke a confrontation with the officers.

Indeed, as the investigation continued into Friday night, a leading theory described an instance of simple, yet tragic, clumsiness on the part of the officer. Mr. Gurley was not armed, the police said.

The episode promised to bring scrutiny to a longtime police practice of officers drawing their weapons when patrolling stairwells in housing projects.