Two sailors were killed Wednesday in a helicopter crash off the coast of Norfolk during a training mission, Navy officials said.

Navy and Coast Guard personnel took four members of the helicopter’s crew to a nearby hospital, and two of them died later. Rescuers were still searching for the fifth crew member late Wednesday.

The Navy said in a statement that the MH-53E Sea Dragon aircraft crashed during a “routine training operation.”

Of the two crew members being treated at Sentara Norfolk General Hospital, one was listed in serious condition.

The cause of the crash was not immediately known. The aircraft was part of Helicopter Mine Countermeasures Squadron 14, which is based at Naval Station Norfolk Chambers Field.

The incident came a day after a U.S. Air Force helicopter crashed in Britain, killing four service members. The HH-60G Pave Hawk crashed in an uninhabited area during a training mission along England’s North Sea coast.