It happened over 13 chaotic minutes.

From a 911 call to the gunshot that mistakenly killed a police officer at the hands of another, the events of March 12 unfolded rapidly and ended in tragedy.

This story is based on dozens of interviews with witnesses and law enforcement officials, and on records of police radio calls.

Patrons inside Johnny McGorey’s Irish Pub in Massapequa Park see a young man dressed in black leather, a mask over his face, run a knife along a window of the bar. A few walk outside to Front Street and see the man scrape the blade along a line of parked cars.

It is 8:10 p.m. when a woman at the bar calls 911.

Two blocks east on Front Street, the man jumps on the hood of Theresa Kelly’s Volkswagen sedan as she backs out of her driveway.

The man, two knives strapped to his chest, taps a blade on the windshield and lets out “”a good laugh,”” remembers Kelly, who was leaving her house to visit her sister in Seaford for dinner.

It is 8:14 p.m. when she dials 911 on her cell phone.

Kelly, 71, also calls her son, who is inside their home. Theresa Kelly pulls out of her driveway and follows the man east on Front. Sean Kelly throws on a pair of sneakers and dashes outside, running after his mother along Front, toward Fourth Avenue.

Sean Kelly, 43, recalls later that it felt as if events were already spiraling out of control.

“”Things were happening so fast,”” he said. “”Your head was spinning. It was surreal.””

Responding to police dispatchers, two Seventh Precinct patrol officers spot the man – Anthony DiGeronimo, 21 — on Front Street. They jump from their cars and order him at gunpoint to drop his knife. He hesitates and briefly taunts the cops with his weapon raised before turning and sprinting toward his home a few dozen steps away.

At 8:16, the officers report on their police radio that the man has entered the small, one-story house on the corner of Front and Fourth Avenue. DiGeronimo shares the home with his parents.

The two officers follow the suspect inside the house, where they are met by DiGeronimo’s mother and father, Joanne and David. The couple beg their son to leave his locked bedroom at the end of a hallway; the officers demand he come out.

Outside the home, a civilian pulls up to the curb. John Cafarella, 58, spent 26 years as an officer with the New York Police Department, most of them with a unit that handled high-risk situations. Now retired, Cafarella was driving to dinner with his wife when he saw the officers’ initial confrontation with DiGeronimo and the brief chase to the house.

Cafarella calls 911 and tells the dispatcher that he will get out of his car, get the address of the home, and report back.

Inside the house, DiGeronimo opens his bedroom door to flash an obscene hand gesture at police before slamming it shut. The officers, faced with a barricaded suspect, move the parents to another area of the house.

“”Please don’t shoot my son!”” shouts David DiGeronimo.

It is 8:17 as members of the Bureau of Special Operations, the Nassau Police Department’s chief tactical unit, report on the radio that they are responding.

Among the unit’s officers who head to the scene are Geoffrey J. Breitkopf, 40, and his partner of nine years. The two men had been on a robbery detail in a neighboring precinct when they picked up radio reports of events at the DiGeronimo home.

Inside the house, the officers call for backup.

Moments later, DiGeronimo rushes down a hallway at the officers, investigators later say, a knife held high above his head. Both open fire at close range, one officer with four shots, the other three, striking DiGeronimo in the chest. He falls in the hallway, dead.

At 8:19, a report goes out over the police radio: Shots fired; an ambulance is requested.

At 8:21, a 911 dispatcher advises officers rushing to the scene to “”slow down,”” indicating the situation at the home is under control.

DiGeronimo’s father, inside the house, can be heard by people on the street screaming hysterically. Outside, Cafarella watches as police cruisers converge at the corner. As they do, Sean Kelly, trying to reach his mother parked farther along the block, argues with an officer, who’s telling him to stay back.

He told the officer to arrest him if he wanted. “”My mother has just been attacked,”” Kelly said. “”I’m going to her.””

As many as a dozen Nassau patrol cops pull up as neighbors congregate on the street.

In the swirl of activity are two Metropolitan Transportation Authority officers who’d traveled four blocks from the Long Island Rail Road station, where they had gone to check on a report of a stuck elevator. One of them is Glenn Gentile, 33, an MTA cop since 2006.

Heeding an order to leave the area, Sean Kelly turns around to head home, saying to another officer, “”Someone just pulled a knife on my mother.””

“”You have nothing to worry about,”” the officer tells him. “”The kid is dead.””

Nearby, Breitkopf, a solidly built man with a goatee, arrives and parks on Front Street. His vehicle is unmarked and, like all Bureau of Special Operations officers, he is wearing plain clothes, jeans and a dark hoodie.

He takes an M4 assault rifle from among the gear loaded in his car. He does not retrieve his armored vest or a jacket emblazoned with a police insignia. He is wearing a badge hanging on a lanyard around his neck. Later, it will be unclear whether he had worn it over or under his clothing.

Breitkopf walks past several uniformed cops toward the house, the black weapon on a sling around one shoulder, the barrel pointed down. Collecting himself nearby is one of the cops who minutes earlier had shot DiGeronimo.

As Breitkopf crosses the lawn toward the front door, someone shouts an alert about a man with a gun. Police union leaders say it was Cafarella, the retired NYPD officer.

As Gentile’s partner grabs Breitkopf by the arm, Gentile draws his sidearm and, from a few feet away, fires a shot without a warning.

Sean Kelly, walking on Front Street toward home, hears the pop. “”I thought, ‘What was happening, why is there another shot? I thought the kid was dead.’ “”

Breitkopf falls face down on the lawn, shot through the heart and lungs. The first of a series of radio calls goes out reporting that a member of the force is in immediate need of an ambulance.

It is 8:23.

Who was involved

—Nassau Police Officer Geoffrey J. Breitkopf, 40, of Selden. Breitkopf joined the department 12 years ago and became a member of the Bureau of Special Operations in 2003. He is the department’s first friendly fire fatality since 1976.

—Anthony DiGeronimo, 21, of Massapequa Park. Police say DiGeronimo menaced several people with knives before rushing at officers in his home, where he was fatally shot.

—John B. Cafarella, 58, of Massapequa Park. He joined the NYPD in 1982 and retired as a sergeant in 2008. Most of his police career was spent in the NYPD’s Emergency Services Unit.

–MTA officer Glenn Gentile, 33, of Holtsville. He grew up in Massapequa and joined MTA police in 2006. His father, Roger Gentile, was a Nassau police detective who died in 2007.