Military helicopters and ships scoured the waters off San Diego on Friday for nine people involved in a midair collision between a Coast Guard plane and a Marine Corps helicopter.

After searching most of the day, rescue teams did not find any survivors or bodies, only a field of debris floating in the Pacific. Military officials said they did not know how the crash had occurred, or which aircraft had struck the other, saying they were focused on search efforts.

Seven people were aboard the Coast Guard’s C-130 transport plane and two were in the Marine Corps’ AH-1 Cobra helicopter when the crash occurred Thursday at 7:10 p.m., officials said. They were flying 15 to 25 miles east of San Clemente Island, a narrow strip of land off the Southern California coast used by the Navy for training exercises.

Right before the crash, air traffic controllers with the Federal Aviation Administration told the Coast Guard pilot “to contact military air traffic controllers,” said Ian Gregor, an F.A.A. spokesman.