The shooting on Monday erupted just after 9 p.m., when, the police said, Leroy Webster, 32, opened fire on Mr. Johnson, 29, in a dispute that escalated after an earlier confrontation the men had on Park Place, blocks away from the path of the West Indian Day Parade.

As Mr. Webster fired several shots, the police said, Mr. Johnson was struck in the neck and fell mortally wounded.

Several officers confronted Mr. Webster, who officials said ignored their orders and fired at them. Eight officers returned fire — in two distinct volleys — firing 73 bullets and striking Mr. Webster twice, in his chest and hip....

Asked if a bullet fired by a police officer could have been the fatal shot, Paul J. Browne, the Police Department’s chief spokesman, said it could have been. “Was it one of the officers’ Glocks? Possibly,” he said. “Was it from the gun witnesses said Mr. Johnson had, but has not been recovered? Possibly.”

The ballistics tests determined that the bullet that killed Ms. Gay, from a 9-millimeter gun, had markings consistent with those made by seven manufacturers’ guns, including Glock.

It was doubtful whether the source of the shot that killed Ms. Gay would definitively be determined, even by comparing the slug to those from the Glocks fired by two officers, the police said, because of the generic markings the lead slug picked up as it passed through the gun’s barrel. Also, the round retrieved from Ms. Gay’s body was deformed.