A veteran NYPD detective nearing retirement was shot in the back during a chaotic early-morning confrontation with an armed 18-year-old in Brooklyn’s Crown Heights — but investigators believe the bullet came from one of his fellow officers, law enforcement sources told The Post.

Detective Robert Karroll, a married father of three just 10 days from retirement, was inside an unmarked police SUV with three uniformed officers around 4:15 a.m. Sunday when the suspect approached the vehicle brandishing a gun. Three officers opened fire, missing the teen but striking Karroll, whose bulletproof vest prevented potentially fatal injuries.

The incident unfolded moments earlier outside a Nostrand Avenue deli. Surveillance video showed the suspect, described as hyped-up, waving a 9mm SAR handgun. He pointed it at others, entered the store, then fired at a passing Uber without hitting anyone. He then circled the officers’ vehicle with his gun drawn, prompting the response.

The unmarked SUV was riddled with bullets in the exchange. A female officer suffered minor injuries. The suspect fled but was arrested blocks away after resisting and being tased. He faces pending charges and has no prior arrests in New York City, though he has a shoplifting record in New Jersey.

Karroll was released from the hospital hours later and is expected to make a full recovery. The NYPD’s Force Investigation Division is examining the shooting.

The episode highlights the split-second dangers officers face in high-risk encounters and the complexities of distinguishing threats amid rapid gunfire. Authorities continue to investigate the full sequence of events.