There are simple protocols familiar to all law enforcement officers meant to prevent one cop from mistakenly shooting another.

It's not known whether such protocols were followed before the chaotic confrontation in Seaford Saturday that left an off-duty federal agent dead, but experts acknowledge that such practices are not fail-safe.

Sources briefed on the investigation said the fatal round was believed to have come from the gun of a retired Nassau police lieutenant.

"Sometimes even when cops do everything right," said Wayne Fisher, director of The Police Institute at the Rutgers School of Criminal Justice, "tragedy is still the outcome."

Before an off-duty or plainclothes officer intervenes in a crime, common protocols advise seeking cover to assess the situation and calling 911. Showing a badge and clearly identifying oneself as law enforcement while approaching is also considered a basic practice.

Investigators are still piecing together events surrounding the shooting of off-duty federal Bureau of Alcohol, Tobacco, Firearms and Explosives agent John Capano as he tried to stop a pharmacy stickup. An off-duty NYPD officer and the retired Nassau lieutenant, both summoned from a nearby deli, came upon Capano and the suspected robber.

Just moments before, Capano, who had been filling prescriptions, pursued the suspect out of the door of the pharmacy after having shot him in the hip or leg.

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The off-duty NYPD officer and retired lieutenant, by some accounts, identified themselves as police. One of them -- it's not clear who -- reportedly went for Capano's gun. Sources said a round fired from the retired lieutenant's gun tore through Capano's aorta and killed him.

The robbery suspect, ex-convict James McGoey, 43, of Hampton Bays, who had brandished a pellet gun that resembled a firearm during the holdup, went for Capano's gun after it had been knocked loose, sources said. The off-duty NYPD officer fatally shot him, according to sources.

Jon Shane, an assistant professor at John Jay College of Criminal Justice, who advised a state committee that looked at police-on-police shootings in 2010, suggested that sheer proximity among the people as such cases unfold is often critical.

"If I am across the street and I have distance and cover and concealment, I can assess and make commands all I want," Shane said. "But if I am in a business right next door to you, I don't have the luxury of time and distance that I would have if I am across the street."

The report produced by the committee that Shane advised noted that the firing of a gun, as Capano had done moments before being confronted, can lead to "auditory block," the inability to process verbal commands.

Fisher, the director of the Police Institute at Rutgers, said literature on such confrontations suggests that even before a gun goes off, stress can alter sense perceptions.

Jim McNally, an ATF spokesman, called the events surrounding his colleague's death "a perfect storm" and "the worst-possible scenario."