The incident was reported at Park Avenue and East 122nd Street around 11:30 a.m.
A 29-year-old man who works in the area was walking when a tan SUV pulled up alongside him and two men wearing all black and face masks got out and robbed him at gunpoint.
An off-duty detective with the NYPD of the Firearms Suppression Division witnessed the incident, identified himself and drew his service pistol while ordering the suspects not to move.
One of the suspects turned toward the detective with a firearm in his hand, which police say prompted the officer to fire one shot.
The suspects dropped their pistol and ran away before getting back into the SUV.
That vehicle has since been identified as a tan, GMC Terrain. It has Florida license plate 8814 AR, and it was last seen traveling over the Willis Avenue bridge into the Bronx.
The suspects took cash and a wristwatch from the victim. It's unclear at this time if the victim was targeted.
Police are searching for the two suspects and the driver of the vehicle.
The off-duty officer was taken to the hospital with tinnitus.
The victim was not injured. It is not yet clear if any of the suspects were shot.
No arrests have been made and the investigation is ongoing.
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