Mr. Bratton said that officers had been seeking Mr. Blackwell, who lives a block away from the site of the shooting, on 104th Avenue, to speak with him in connection with a crime, though whether he was a suspect or a witness was not immediately clear.

But it was “activity he engaged in” that drew the attention of the officers on Saturday, Mr. Bratton said, specifically the object in the waistband.

Officer Moore appeared to have been shot in the left cheek and the bullet went out the right side of his head, toward the back of the head, a law enforcement official said, speaking on the condition of anonymity to discuss the fast-moving events. The wound, then, could be what is known as an “in-and-out,” the official said, meaning it might have missed critical organs, his survival a matter of inches.

Dr. Eli Kleinman, the supervising chief surgeon of the Police Department, said at the news conference that quick work by the first responding patrol car and a team of neurosurgeons had helped them confront a “life-threatening situation.”

After the shooting, officers could be seen going through the garbage outside the white-paneled home with red steps where Mr. Blackwell lives. As of late Saturday, the gun used in the shooting had not been found.

Image Mr. Blackwell, 35, in a photograph from the Police Department's Twitter account, was arrested on Saturday.

A cousin of Mr. Blackwell’s who lives near him but who declined to give her name said in a brief telephone interview late Saturday that she was just learning about the shooting: “I am just finding all this stuff out myself. I don’t know anything. All I can do is pray right now.”