The officers opened fire and dozens of rounds shattered the autumn night. When it was over, Mr. Prioleau lay mortally wounded, a 9-millimeter pistol near him, and the man he had apparently targeted had fled.

Chief Terence A. Monahan, the Police Department’s highest-ranking uniformed officer, said that the officers, whom he did not name, “immediately identified themselves as police and gave multiple orders to drop the weapon.”

Mr. Prioleau not only did not comply, but he pointed his pistol at the officers, the chief said.

“The perpetrator was discharging his gun at somebody else,” Chief Monahan said. “The cops happen to be there when he did it. So he turned his gun on them.”

It remained unclear whether Mr. Prioleau had fired his weapon at the officers. The police were still looking for the second man on Wednesday, officials said.

The officers were members of an anti-crime unit, an elite, aggressive team whose main mission is taking guns out of criminals’ hands. Anti-crime units often respond to reports of people armed with guns, or to sounds of gunfire.