(CNN) -- Authorities have decided to call off a search-and-rescue mission for nine people who may have plunged into the Pacific Ocean off southern California after a Coast Guard C-130 plane and a Marine AH-1 Cobra helicopter collided Thursday night.
"I've reached the conclusion that hope is no longer viable," Coast Guard Rear Adm. Joseph Castillo said Sunday. "We no longer believe there is any chance somebody could still be alive."
California Gov. Arnold Schwarzenegger offered his condolences to family members of the missing Sunday.
"Together, we send our thoughts and prayers to their families and friends," Schwarzenegger said in a statement. "Our hearts are with them during this difficult time."
Searchers had scoured the ocean for 60 hours without finding any sign of survivors. They reported Saturday that the search had covered 644 square miles. The Navy, Marine Corps and Customs Border Protection helped in the search.
The Coast Guard weighed the exhaustive nature of the search, the wreckage, the nature of the collision, the temperature of the water and the time elapsed. A Coast Guard captain said Friday that survivability could be up to 20 hours.
On Saturday night, relatives of the missing were briefed about the decision to call off the rescue effort. The order was to go into effect as soon as the planes involved in Sunday morning's dawn search returned about 9 a.m.
Coast Guard pilots had been searching for a missing person in the water. That person was "reported to have gotten in a dinghy and attempted to row to Catalina [Island]," Coast Guard Capt. Thomas Farris said Friday. "We were searching in that area because of the drift that would have naturally occurred after that event."
The collision occurred about 7 p.m. Thursday, when the Coast Guard plane with a seven-person crew collided with a Marine AH-1 Cobra helicopter carrying two people.
A pilot not involved in the incident reported seeing a fireball about 7:10 p.m. Thursday near the crash site.
The two Marine pilots were conducting routine training about 15 miles off San Clemente Island when their helicopter collided with the U.S. Coast Guard plane.
Marine Corps Air Station Miramar identified them as Maj. Samuel Leigh and 1st Lt. Thomas Claiborne, both with Marine Light Attack Helicopter Squadron 469.
The Coast Guard's seven missing personnel were stationed at Coast Guard Air Station Sacramento, California, where their aircraft was based.
Castillo has said that an investigation was beginning with the Marine Corps. A large debris field has been located, and pieces have been collected, the Coast Guard spokesman said.
The Coast Guard identified its seven missing personnel as Lt. Cmdr. Che J. Barnes of Capay, California, aircraft commander; Lt. Adam W. Bryant of Crewe, Virginia, co-pilot; Chief Petty Officer John F. Seidman of Stockton, California, flight engineer; Petty Officer 2nd Class Carl P. Grigonis of Mayfield Heights, Ohio, navigator; Petty Officer 2nd Class Monica L. Beacham of Decaturville, Tennessee, radio operator; Petty Officer 2nd Class Jason S. Moletzsky of Norristown, Pennsylvania, air crew; and Petty Officer 3rd Class Danny R. Kreder II of Elm Mott, Texas, drop master.
The Coast Guard announced that a memorial service for personnel missing in the collision will be at 11 a.m. PT Friday at the Coast Guard station in Sacramento.