Two police officers in an emergency truck plunged more than 40 feet off an open drawbridge in thick fog. One was killed; the other was missing and feared dead.

The vehicle fell into the Hackensack River on Sunday night, after the officers crossed the Lincoln Highway Bridge and placed flares to warn motorists that the bridge's safety warning system was not operating, said Police Chief Robert Troy.

Before the officers turned around and drove back across the river, the bridge's middle span was raised to allow a tugboat to go under.

"They dropped off the cones and the flares, wished everyone a Merry Christmas and were joking around. From what I've heard they were all in good spirits," Mayor Jerramiah Healy said.

"The horrible irony is they were responding to the very situation that caused their demise. The bridge operator wanted cones and flares and our police department was the first to respond."

The safety bar and bell used to warn motorists when the bridge is open had not been working for two days because a vehicle had crashed into them, officials said.

The body of Officer Shawn Carson, 40, was found Sunday night. Carson was a 16-year veteran of the force. Divers searched the 40-degree water Monday for Officer Robert Nguyen, 30, who had been with the police department for six years.

The bridge, which links Jersey City and Kearny, is known by several names, including the Hackensack River Bridge. It is run by the state Department of Transportation as an "on-demand bridge," meaning operators respond to requests from boat pilots rather than following a set opening schedule.

Officials said it is raised about 30 to 40 times a month, depending on the time of year and commercial boating activity.

Department spokeswoman Erin Phalon said investigators were trying to determine whether the bridge operator was required to contact the officers before they crossed back into Jersey City, or if the officers should have notified the operator.

"Everything in the universe that could have gone wrong, did go wrong," said police Lt. Tom Comey.