Rescue crews searched Thursday for survivors after a Marine Corps helicopter with four people aboard plunged into the Pacific Ocean 12 miles off the Southern California coast.

Officers from Camp Pendleton Marine Base would not say why or where the UH-1 "Huey" crashed shortly after 8 p.m. Wednesday. One official, who declined to give his name, confirmed only that the helicopter was assigned to the base 30 miles northwest of San Diego.On Sept. 4, two Air Force HH-60G Pave Hawk helicopters crashed in the Nevada desert during a training exercise. All 12 people on board were killed.

The UH-1, made by Bell Helicopter Textron, became a military mainstay as they ferried American troops in and out of battle during the Vietnam War.