A U.S. Navy helicopter crashed Friday near San Clemente Island in the Pacific Ocean, killing at least one of the four people aboard, a Navy spokesman told CNN. Cmdr. Jack Hanzlik said the MH 60 Sierra Seahawk helicopter was conducting a routine training mission off the USS Bonhomme Richard with four aboard. It crashed at 2:30 p.m. (5:30 ET). Search and rescue efforts are under way for the other three, he said. He said the cruiser USS Chosin and destroyer USS Chung-Hoon were participating in the exercise and the rescue effort. The Coast Guard is also assisting in the rescue effort. No other aircraft were involved in the mishap, Hanzlik said.