John Joseph Staus, 12, wanted to be a police officer some day like his stepfather, William R. Trotti, an 11-year veteran of the Alexandria police force. The boy sometimes rode along with Trotti in a city police cruiser, his mother said, and would go to court to hear him testify. He planned to return to police youth camp this summer.

On Tuesday morning, John Staus, holding a toy gun, was playing cops and robbers with his stepfather 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 recently carried while on duty, police said.

As the boy imitated the sound of a gun firing, Trotti turned toward him. In that instant, the service revolver discharged, according to the police account of the incident. A single, hollow-point bullet struck John Staus in the chest.

The boy, who would have begun the seventh grade this fall, was declared dead at Alexandria Hospital at 10:54 a.m., about an hour after he was shot. Trotti, 32, described by a police colleague as deeply in shock, was placed on administrative leave. The shooting is under investigation, but preliminary findings are that it was an accident.

"I know and John knows how much his stepdad loved him. John would be the first to forgive the accident," said Susan Staus Trotti, 32, the boy's mother, who was in the apartment when the shooting occurred along with John's maternal grandmother.

"We have lost a jubilant and a lively boy. He was a gift that made life richer than it would have been without him," Susan Trotti said.

Deputy Alexandria Police Chief Arlen Justice said yesterday that the investigation has shown so far that when Trotti turned toward his son, he thought that he had emptied all the bullets from his weapon. But, Justice said, "he had inadvertently left one in."

"They were just doing one of those horseplay kind of things," Justice said of the incident that led to the boy's death.

An autopsy is being conducted by the Northern Virginia Medical Examiner's Office to determine whether the bullet that struck John Staus ricocheted or entered his body directly, Justice said.

The weapon involved in the incident Tuesday had been given to Trotti by the police department as a replacement for his own service revolver, which had been taken for examination after another shooting incident a month ago. There were no injuries in that incident.

The previous shooting incident occurred at a small manufacturing firm at 2 a.m. on June 2 in Alexandria, when the owner apparently thought that he was being stalked by burglars. According to police, the owner of the business, James H. Nachod, 39, opened fire with a handgun after he saw figures outside his office door. Gunfire was returned by police officers who had gathered at the building after the office door was found ajar. One of the police officers who fired his gun was Trotti, police said yesterday.

According to Deputy Chief Justice, an internal police investigation after that incident determined that the police response was justified.

Nachod, whose business had been burglarized twice before the incident, was charged with attempted felonious assault and was released on a $1,000 personal bond.

On Tuesday, when the replacement revolver discharged in the family apartment, Trotti had been preparing the weapon to return it to the department, since the investigation of the June incident had been completed.

Justice said he expects that a joint investigation into Tuesday's incident by the police department and the Alexandria Commonwealth's Attorney's Office will be completed within the next few days.

Alexandria police officers are not bound by any specific rules on how they must store or secure their guns at home, but all received training in gun safety, Justice said. Officers are encouraged to lock guns up, keep them separate from ammunition, or place their handcuffs through them so that the cartridge cylinder cannot be put into a firing position, a police department spokesman said.

William Trotti could not be reached for comment yesterday. Meanwhile, Susan Trotti said that her husband's career had enriched her young son's life.

The child's rides in the police cruiser with his stepfather were "some of the most thrilling experiences in his life," Susan Trotti said. John, a student at Terra-Centre Elementary School, knew most of Trotti's fellow police officers by name and was captivated by his stepfather's profession.

"It was the most loving, warm relationship I've ever seen between two people," Susan Trotti said of her husband and son.