U.S. Coast Guard divers board a MH-60 Jayhawk helicopter early Friday. They were searching for as many as nine people off the San Diego coast following a mid-air collision. (Associated Press)

Investigators are trying to determine why a U.S. Coast Guard C-130 airplane and a U.S. Marine Corps attack helicopter collided on Thursday night, likely killing nine crew members.

A C-130 similar to this one is believed to be missing. The U.S. coast guard plane collided with a military helicopter off the Southern California coast Thursday. (Donald Knowles/Associated Press)

Aircraft and ships are scouring the ocean off San Diego for any signs of survivors of the nighttime collision, but Pentagon spokesman Bryan Whitman said the crash likely killed both the plane's seven crew members and the two-person crew of the AH-1W Super Cobra helicopter.

Federal Aviation Administration spokesman Ian Gregor said the crash occurred minutes after control of the C-130 was handed off from FAA controllers to military air controllers.

Levi Read, a coast guard petty officer, said the plane was conducting a nighttime search for a boater. Military officials said the helicopter was one of four marine choppers flying in formation to San Clemente Island, a Navy training site.

The collision occurred 80 kilometres west of the San Diego County coast and 24 kilometres east of the training site.

With files from The Associated Press