An Alexandria police officer who accidentally shot and killed his 12-year-old stepson during a game of cops and robbers resigned from the department last week.

Police Chief Charles T. Strobel said yesterday that Officer William R. Trotti's resignation on Aug. 8 ended an internal review of the shooting, which was nearing completion.

Strobel said Trotti, 32, a member of the force for 11 years, didn't submit any reasons for his resignation.

Trotti, who had been on administrative leave since the July 5 shooting, had been expected to return to his duties after an Alexandria Circuit Court grand jury declined to indict him Aug. 1 on a charge of involuntary manslaughter.

The chief said previously Trotti was receiving medical attention as a result of the shooting, and the department was awaiting clearance from his doctor before returning the officer to duty.

The shooting occurred as Trotti's stepson, John Joseph Staus, was holding a toy gun and playing cops and robbers with the officer in the living room of the family's 10th floor apartment on Duke Street.

Trotti was standing nearby, inspecting a .38-caliber service revolver that he had recently carried while on duty, police said. As the boy imitated the sound of a gun firing, Trotti turned toward him, and the revolver discharged, according to police. A single, hollow-point bullet struck Staus in the chest. He was pronounced dead at Alexandria Hospital a few minutes later.

Deputy Police Chief Arlen Justice had said that Trotti believed his gun was empty when pointed toward his stepson. "They were just doing one those horseplay kind of things," Justice said.