No one was struck by the gunfire in an underground parking garage on 101st Street in Bay Ridge just before 6 a.m.
Police say the 57-year-old retired officer got into a verbal dispute with a 54-year-old man, and that the suspect lunged at the retired officer and punched him in the face.
The retired officer pulled his weapon and fired a round, striking a car.
The officer, who retired in 2007, was treated at NYU Langone Hospital-Brooklyn for minor injuries.
The suspect was taken into custody at the scene.
Charges are pending.
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