Half Moon Bay, California - A small private plane crashed Saturday in the Pacific about 40 miles west of Half Moon Bay, California, following an emergency call for help, killing two people aboard.

The National Transportation Safety Board said the plane was a Viking Air DHC-6-400 Twin Otter, headed from Santa Rosa-Sonoma County airport to Honolulu, Hawaii.

Two hours later, it was forced to turn around due to what the pilot told air traffic controllers were "mechanical issues," a US Coast Guard spokesman said.

The small turboprop plane is listed on aviation sites as typically having a range of about 700 miles, but FAA records show the plane had been outfitted with a tank system to allow longer flights.

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According to the National Transportation Safety Board, the plane took off from Santa Rosa about 8:20 AM PT, but turned back around 1:15 PM, headed for a small public airstrip at Half Moon Bay and eventually crashing.

The US Coast Guard out of San Francisco broadcast an alert at 1:40 PM of a "small plane in distress" southwest of the Farallon Islands and advised mariners to look for people in a yellow life raft. But when a US Coast Guard helicopter reached the location, the crew found the plane upside down in the water, its occupants still inside, said Petty Officer Matthew West.

National Transportation Safety Board spokesperson Sarah Taylor Sulick said there were no other passengers on board, and that NTSB investigators are working to recover the plane.

The identities of the pilot and co-pilot have not been released. The NTSB said it was still determining ownership of the plane. FAA records show it was registered to a trust managed by the Bank of Utah.