“No one heard, ‘Stop! Police!’ ” James Carver, the president of the Nassau County Police Benevolent Association, said.

However, someone seems to have yelled something: officers have said that a civilian at the chaotic scene — possibly a retired New York City police sergeant — was heard yelling “Gun! Gun!” or words to that effect just before Officer Geoffrey J. Breitkopf, who was in plain clothes and carrying a rifle, was shot on Saturday night, Mr. Carver said.

If the account is accurate, it adds a member of a third police department, albeit retired, to the scrum of officers outside a crime scene where a lack of recognition among officers proved fatal. The officer who fired the fatal shot was from the Metropolitan Transportation Authority’s police department.

The union’s account of the shooting of Officer Breitkopf, 40, a 12-year veteran of the Nassau County force, also suggests that his fellow Nassau officers knew he was a police officer but that the transit agency officer who shot him, Glenn Gentile, 33, did not.

The shooting occurred outside a home in Massapequa Park where, a few minutes earlier, a deranged man, Anthony DiGeronimo, 21, had reportedly lunged at Nassau County officers with knives after a neighbor called 911 to report that he had threatened her on the street. Officers shot and killed him in his home in what the Police Department called self-defense.

Detective Vincent Garcia, a spokesman for the Nassau County Police Department, which is investigating the shooting of Officer Breitkopf, said he could not confirm the union president’s account.

Officer Gentile, 33, has been with the transit agency’s police force for five years, and his father, Roger, who died in 2007, was a Nassau police detective. Officer Gentile had been at a nearby Long Island Rail Road station with a partner when they heard the call about the situation on the Nassau County police radio frequency and responded. Interagency shows of support are not uncommon and go in both directions.

Officer Breitkopf, a member of the Nassau police’s Bureau of Special Operations, which responds to shootings and other violent situations, arrived with his partner in an unmarked car about 10 minutes after the shooting of Mr. DiGeronimo and emerged from it, in plain clothes, carrying an M4 rifle, Mr. Carver said. The officers had radioed ahead to announce their arrival on the same radio frequency that the transit agency’s officers had been monitoring, Mr. Carver said.

Officer Breitkopf, who was wearing a badge on a chain around his neck, exchanged pleasantries with Nassau County officers across the street from the DiGeronimo house and said he was going to go up for a look, Mr. Carver said.

It is unclear where Officer Gentile was when Officer Breitkopf arrived.

According to Nassau County officers at the scene, Officer Breitkopf was wearing his rifle on a sling around his shoulder, its barrel pointed down along his right side and his hand against it to keep it from banging, Mr. Carver said.

“He doesn’t have his finger on the trigger, obviously, but he has his hand on the rifle to secure it to keep it close to his body,” Mr. Carver said.

via Cry of ‘Gun!’ Is Claimed in Fatal Shooting of Nassau Officer – NYTimes.com.