San Diego ? The nighttime collision of a Coast Guard aircraft on a rescue mission and a Marine helicopter left nine people feared dead at sea Friday as investigators tried to solve the mystery of how the aircrews failed to see each other in a heavily used military training area.

Military aircraft and ships searched the ocean off Southern California for any sign of the victims while investigators gathered recordings of air traffic controllers and pilot communications. The search covered 644 square miles of ocean but focused on a debris field 50 miles off the San Diego coast.

The crash involved a Coast Guard C-130 with a seven-member crew and a Marine Corps AH-1W Super Cobra with two aboard as it flew in formation near the Navy’s San Clemente Island, a site with training ranges for amphibious, air, surface and undersea warfare. It was not known whether the pilots were aware of each other before the 7:10 p.m. Thursday collision.

“A tragic event,” Pentagon spokesman Bryan Whitman said. “The search is still on, but it’s likely taken the lives of nine individuals.”

In San Diego, crews of Navy ships, Coast Guard cutters and helicopters planned to keep scouring the ocean even though nearly a full day had passed since the accident.

“We’re still in the search-and-rescue phase, we are not standing down from that at this point,” said Capt. Tom Farris, commander of the Coast Guard’s San Diego sector. “We have every hope we will find survivors.

The identities of the crew members were not immediately known. The C-130 crew had survival gear aboard the aircraft, including exposure suits that could have allowed them to survive in the water for hours, Petty Officer Henry Dunphy said.

The Sacramento-based C-130 crew was looking for a man on 12-foot motorized skiff who was reported missing after leaving Avalon Harbor on Santa Catalina Island to reach a friend on a disabled yacht that had gone adrift off Catalina in high winds Tuesday, authorities said.

The Marine helicopter was flying from Camp Pendleton near San Diego to San Clemente Island, said Maj. Jay Delarosa, a spokesman for Marine Corps Air Station Miramar in San Diego.

Two Super Cobras, a type of attack helicopter, were escorting two big CH-53E Super Stallion transport helicopters carrying Marines to the island, Delarosa said. He did not know how many Marines were aboard the transports.

After the collision, the other three helicopters returned to base, he said.

The accident occurred in airspace uncontrolled by the FAA and inside a so-called military warning area, which is at times open to civilian aircraft and at times closed for military use, said Federal Aviation Administration spokesman Ian Gregor. He added that he did not know the status of the airspace at the time.