For more than 36 hours after a New York City police officer was shot and killed while wrestling with an armed man in the Bronx, officials could not answer a crucial question: Who fired the fatal shots?

At first it appeared that the officer, Brian Mulkeen, had been shot with his own gun while grappling with the man, who was then killed by responding officers, the police said. The officer's weapon was fired five times, but a .32-caliber revolver found near the man had not discharged a single shot.

But on Monday, the police commissioner said Officer Mulkeen had retained control of his weapon and had fired five shots at the man. Officer Mulkeen, 33, was mortally wounded by his own partners , who according to police radio transmissions mistakenly believed they were being ambushed .

It was the second time in seven months that an officer in the New York Police Department was killed in a fusillade by fellow officers. The back-to-back tragedies prompted questions about the department’s tactics and training as its officers continue aggressive efforts to drive down crime by making gun arrests and dismantling violent gangs.