A Brooklyn man is on life support after a fellow tenant at his apartment building, a retired cop, shot him in the head outside his home Monday afternoon, police sources say.

The alleged shooter, a 51-year-old police officer who retired in August 2002, shot Joe Stepinski outside the three-family home on Greenpoint Avenue, then went inside the building as police responded late Monday afternoon, law enforcement sources tell News 4.

Responding officers found the 45-year-old victim lying on the sidewalk, police said. He was taken to Bellevue Hospital in critical condition from a gunshot wound to the head, and was on life support Monday night.

A local business owner says she heard the initial gunshot, which she originally thought was a popped tire. Police flooded the area within about a minute.

"Shortly after that, we saw a police officer go and knock on a window at 185 Greenpoint, and we saw the guy come out with a gun, and at that point we all hit the ground," said Camille, the owner of Kween Kargo Bike Shop on Greenpoint Avenue.

Camille said she saw police taking positions behind cars.

"I remember a police officer yelling, 'He has a gun, he has a gun, get back!'" said Jared Friedman, assistant manager at nearby Kimchee Market.

Camille said "at that point me and everybody in the shop hit the floor and waited for instructions from police, and after about 10 minutes, they asked us to kind of crawl out, keep low and kind of come down the block here."

Dozens of police cars swarmed the neighborhood as police responded, including at least one armored vehicle, Chopper 4 over the scene showed. Traffic was diverted off McGuinness Boulevard near the Pulaski Bridge.

The suspect has been taken into custody, and was seen being taken into the 94th Precinct police stationhouse. Two guns have been recovered.

Luis Andino, a friend of the man who was in custody, told News 4 that the victim would" stalk him, spy on him, walk into his apartment. These are things he'd complain about."