A small plane traveling from Santa Rosa to Honolulu crashed off the coast of Half Moon Bay Saturday afternoon, killing the pilot and co-pilot inside.

The National Transportation Safety Board (NTSB) said it was investigating after the Hawaii-bound Viking Air DHC-6-400 Twin Otter crashed into the ocean 40 miles away from Half Moon Bay. The pilot and co-pilot inside were killed, according to Mercury News.

The plane had departed Santa Rosa around 8 a.m. Saturday morning en route to Honolulu, flight data shows, according to Mercury News. The pilot had flown it southwest for about two hours over the Pacific Ocean before suddenly turning around and heading back to California.

The pilot reportedly declared a distress signal — a mayday — around 2 p.m. when the plane was near the Half Moon Bay coast, right before the plane went down.

The U.S. Coast Guard discovered the plane 40 miles southwest of San Francisco Saturday afternoon, as per the Mercury News. Authorities were working to recover the bodies amid the wreckage, as the Chronicle reported.

Neither the identities of the pilots nor more details about the cause of the crash have been released, as the NTSB continues to investigate.

“Normally this type of plane cannot travel that far but it was equipped with a ferry tank system that enables it to travel long distances,” NTSB spokesperson Sarah Taylor Sulick told the Chronicle via email. “That is a fairly normal occurrence, according to one of our investigators.”

Feature image of Air Seychelles Viking DHC-6-400 Twin Otter aircraft via Flickr/ under Creative Commons.