They're drilled into plainclothes cops: the use of clear, simple commands such as, "I'm a police officer, don't shoot!" when facing someone with a gun, and "Police, don't move!" when drawing on a suspect.

Investigators aren't saying whether such standard phrases were spoken Saturday night before a Metropolitan Transportation Authority officer mistakenly shot and killed Geoffrey J. Breitkopf, 40, of Selden, a member of the Nassau Police Department's Bureau of Special Operations.

Police said Breitkopf had a rifle at his side as he approached a Massapequa Park house where other Nassau officers had killed a man minutes before.

Plainclothes officers such as Breitkopf are trained to display a badge when approaching other officers, particularly when armed, according to Nassau police union president James Carver. There is no department policy that they wear identifying insignia on jackets when entering crime scenes, he said. A department source said Breitkopf's badge was found, but that it is not known whether he wore it outside his clothing.

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As a member of an elite unit, which typically operates in plainclothes, Breitkopf would have been especially familiar with such protocols and related measures to prevent "friendly fire" shootings, said John LaSala, the Nassau police union's bureau representative.

"That stuff is drilled in their heads," LaSala said, adding that he believed, "Geoff did everything he was supposed to."

Asked whether its officers were familiar with such procedures, the MTA issued a statement Monday that said they "are highly trained professionals who receive 6 months of rigorous training at the NYPD Police Academy and an additional four months in-service training in rail operations."

Anthony DiGeronimo, 21, of Massapequa Park, was shot by two officers Saturday inside a home on Fourth Street after lunging at the officers with knives, police said.

Jon Shane, an assistant professor at John Jay College of Criminal Justice, said a plainclothes officer such as Breitkopf would be expected to display a sign that he was law enforcement.

And the uniformed MTA officer, identified as Glenn Gentile by those familiar with the investigation, would be expected to issue some kind of verbal challenge, such as, "Police, don't move!" before opening fire, Shane said. As the plainclothes officer in that scenario, Shane said, Breitkopf would be required to comply immediately.

Police-on-police shootings are not uncommon, according to a report issued last year by a state commission that investigated the phenomenon after two such high-profile shootings in New York City.

The report found that the police officers most likely to have weapons drawn on them are either undercover or plainclothes.

"It shouldn't be surprising," said Zachary Carter, a former U.S. attorney who was vice chairman of the commission. "When someone happens on the scene in civilian clothes and they are a stranger to responding officers, there's going to be a degree of risk."

With Alfonso Castillo

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and Andrew Strickler