An off-duty Jersey City police officer was carjacked by armed men at a city car wash yesterday and he fired one round at his own fleeing Mercedes, which is still missing, officials said today.

At 3:15 p.m. the 36-year-old officer's car had just come out of the Waterworks Car Wash at Garfield Avenue and Caven Point Road and he was standing beside it while assisting a female patron when the thieves jumped into his running car, reports said.

When the officer tried to stop the carjackers, he saw one of the men holding what appeared to be a handgun and he fired one round, reports said, but the carjackers continued to flee.

The officer then asked a person at the scene for permission to take their vehicle and he jumped in and chased his own fleeing car, but lost sight of it heading east on Carteret Avenue, reports said.

Because the officer discharged his weapon, the incident is being investigated by the Hudson County Prosecutor's Office and the Jersey City Police Department's Internal Affairs Unit, officials said.

"It is very preliminary at this point in the investigation," said Hudson County Prosecutor Edward DeFazio today, noting that instances in which an officer discharges his weapon are always investigated.