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Chilling video footage captured the moment a deranged teenage gunman racks a round into the chamber of a 9mm handgun before running up to an NYPD vehicle and opening fire — leaving a veteran detective shot in the back.

The 18-year-old shooter is seen flashing the weapon as he walks back and forth on Nostrand Avenue in Crown Heights around 4:45 a.m. Sunday, just a half block from an unmarked police cruiser, surveillance video obtained by The Post shows.

The suspect, who has not been identified, walks back and forth along the sidewalk – at one point seen pulling back the slide of the pistol before dashing up the street to the unmarked police vehicle.Alert cops, part of a sex-offender monitoring unit, saw the nut approaching with the semi-automatic weapon and jumped out – sparking a shootout.

NYPD Detective Robert Karroll was shot in the back in the exchange, with his life likely saved by his bulletproof vest before his partner, who suffered minor injuries, subdued the gunman.

Karroll, who sources said is less than two weeks from retirement, is expected to make a full recovery, while the alleged shooter is being questioned by detectives.

This is a developing story. Please check back for updates.