Two U.S. CH-46 Sea Knight helicopters collided early Monday during a routine military exercise in Okinawa, sending one of them crashing into mountainous terrain and killing four Marines, a military spokesman said. Master Sgt. Jake Rodriguez, chief spokesman for U.S. Forces Japan, said names of the four Americans were being withheld pending notification of next of kin. He said the two helicopters were involved in a routine exercise over the Marine Corps northern training area on the east coast of Okinawa's main island when the collision occurred at about 11:15 a.m. (7:15 p.m. Sunday MST). One helicopter plummeted into remote mountains while the other was able to return safely to the Marine Corps Air Station at Futenma. "Details of the accident are under investigation," Rodriguez said.