A Navy report on the crash faulted the Coast Guard and Marine crews for failing to watch out for each other, saying it was their responsibility to avoid each other while flying even though a Navy controller was aware of their presence.
Under "see-and-avoid" rules that govern airspace off the Southern California coast, pilots are on their own to watch for others in the area. The collision occurred about 50 miles off the coast.
"Both aircraft were operating under visual flight rules and were ultimately responsible for their own safety, navigation and separation from other aircraft," the Navy report said.
The Navy acknowledged that a controller at Naval Air Station North Island in Coronado, near San Diego, failed to alert the pilots. It said the controller was giving higher priority at the time to Navy F/A-18 fighter jets in the area.
It recommended that a supervisor be assigned to provide additional oversight.
The Coast Guard report, also released Tuesday, blamed Navy controllers for failing to recognize a formation of four Marine helicopters make a climbing right turn, one of which collided with the Coast Guard plane. It said it was in radio contact with Navy controllers for 2½ hours before the crash, which may have led the Coast Guard crew to believe that it would be warned of any aircraft in the area.
The Coast Guard and Navy both agreed there was no single reason or person to blame for the crash. They each made a series of recommendations to improve safety in the largely unregulated airspace. The Marines also published findings but did not release them Tuesday.
The collision "was the product of a tragic confluence of events, missed opportunities and procedure/policy issues where most aircraft fly under a 'see-and-avoid' regime," the Coast Guard report said. The Navy report used almost identical language.
The Coast Guard plane was on a mission to rescue a boater who had been missing for two days near San Clemente Island, while the helicopter was on a training mission on the night of Oct. 29, 2009. All aboard both aircraft were killed.
The reports mentioned several other factors. The Coast Guard said that said Marine pilots gave its crew "little opportunity" to see them. The Marine helicopter that crashed had not turned on its anti-collision light and transponder.
A Marine Corps spokeswoman, Lt. Maureen Dooley said it was "very difficult" for the Coast Guard crew to see the Marine helicopters because the Coast Guard did not have night-vision goggles. The Coast Guard said its cockpit lighting is incompatible with that equipment.
In defending the helicopter's lack of lights, she said the Marines were following military regulations, a point acknowledged in the Coast Guard report. She said the training exercise that the Marines were on required dim lights and the formation.
The Marines also published findings but did not release them Tuesday.
The collision occurred in a so-called military warning area - airspace that is not controlled by the Federal Aviation Administration. It has ignited a debate within the military branches about how to best police the sky in the giant military training area, which spans 1 million square miles off the Southern California coast.
The Coast Guard, in one of its recommendations, urged the Navy to establish guidelines on the West Coast for communicating with other agencies on search-and-rescue missions, as it does on the East Coast.
"The unfortunate thing is, had any of the parties done anything differently we might have avoided this tragic confluence of events," said Cmdr. Pauline Storum, a Navy spokeswoman.
--- Associated Press writer Michael R. Blood in Los Angeles contributed to this report.