Eight people were confirmed dead from a predawn fire that sank a boat off a Southern California island and 26 others who were on the scuba diving vessel were missing, a local sheriff said.
The fire broke out aboard the Conception, a 75-foot (23-metre) boat, at about 3:15 a.m. while it was moored just off the shore of Santa Cruz Island, the U.S. Coast Guard said in a statement.
Passengers slept in the ship's lower quarters, officials said, while five crew members who were above deck on the bridge escaped.
The Coast Guard searched the coastline of Santa Cruz Island for any other possible survivors but had not found anyone, officials said.
"This isn't a day that we wanted to wake up to for Labor Day and it's a very tragic event, Coast Guard Captain Monica Rochester told a news conference.
"We will search all the way through the night into the morning but I think we all should be prepared to move into the worst outcome," she said.
Four bodies were recovered from the area and rescue divers found another four bodies on the ocean floor near the sunken vessel, which lies upside down under more than 60 feet (18 metres) of water, Santa Barbara County Sheriff Bill Brown told reporters.
"Fire is the scourge of any ship," Brown said, and to be in a remote location, asleep and then have a fire spread very quickly, "you couldn't ask for a worse situation."
The four bodies that were recovered will have to be identified through DNA samples, Brown said.
The Conception, which launched in 1981, embarked for California's Channel Islands on Saturday morning with 39 people on board. Excluding the five survivors and the eight people confirmed dead, search teams are looking for 26 people who were on the Conception, Brown said.
Authorities are trying to determine the best way to recover the sunken vessel, including the possibility of towing it to shore, he added.
The surviving crew members sought refuge on a fishing boat moored a few hundred feet away, banging on the side to wake up Bob Hansen and his wife, who were sleeping onboard.
"When we looked out, the other boat was totally engulfed in flames, from stem to stern," Hansen said in an interview with the New York Times. "There were these explosions every few beats. You can't prepare yourself for that. It was horrendous."
Scuba or propane tanks on the Conception may have exploded in the fire, but that had not been confirmed, Brown said, adding it was unclear if there was an initial explosion that caused the fire.
After borrowing clothes from the Hansens, some crew members headed back toward the Conception to look for survivors without luck, Hansen said.
U.S. Senator Dianne Feinstein, a Democrat who represents California, called for an investigation.
"It's inconceivable that with all the safety regulations we have in place today, a fire on a boat can lead to the loss of life we saw this morning," she said in a statement. "And we need to understand exactly how the crew was trained and, if they were awake and above-deck, why they were unable to alert or help rescue passengers."
Investigators said a single mayday call came from the boat reporting the fire.
"It happened quickly enough so many people could not get off," U.S. Coast Guard Lieutenant Commander Matthew Kroll told CNN.
The diving boat was chartered by Worldwide Diving Adventures, a Santa Barbara, California, excursion firm. It said on its website that the Conception was on a three-day excursion to the Channel Islands, and was due back in Santa Barbara at 5 p.m. on Monday.
The boat's owner, Truth Aquatics, referred queries about the accident to a joint media center. "This is still an ongoing search and rescue," it said.
(Reuters reporting by Omar Younis; Additional reporting by Peter Szekely and Jonathan Allen in New York and Alex Dobuzinskis and Bill Tarrant in Los Angeles; Editing by Nick Zieminski and Peter Cooney)