At least 15 bodies have been found after a 75-foot commercial diving boat on a Labor Day excursion caught fire off the coast of Santa Cruz Island in Southern California, according to Coast Guard officials who spoke to the Los Angeles Times. More than a dozen passengers are still missing.

CNN previously reported eight bodies had been recovered. Santa Barbara County officials told CNN additional victims are being recovered, but could not verify the total number.

Santa Barbara Sheriff Bill Brown said earlier on Monday that four bodies had been recovered and had yet to be identified. Four other victims were spotted at the bottom of the ocean “in close proximity” to the boat, named the Conception, he said.

“Our hearts go out to the families of the victims of this terrible tragedy,” Brown said.

There were a total of 39 people on board – 33 passengers and six crew members, said Coast Guard Capt. Monica Rochester. Twenty-six people are still missing.

“We all should be prepared to move into the worst outcome,” she told reporters.

Of the crew who were rescued, one suffered minor injuries. Four of them were being taken ashore, while the Conception’s captain has opted to remain on the scene, Coast Guard Chief Warrant Officer Josue Mendez said.

Rochester earlier said the vessel was in compliance with regulations.

The cause of the blaze is not clear. Authorities said there is no evidence of a criminal act at this point.

Firefighters had difficulty snuffing out the fast-moving fire, the Coast Guard’s Aaron Bemis told CNN. Each time it was extinguished, it flared back up — perhaps because of the fuel on board, he said.

“The Conception burned down to the water line,” said Santa Barbara County Fire Department spokesman Mike Eliason.

Foggy conditions also complicated the rescue operation.

The boat was about 20 miles off the mainland coast, near Santa Cruz Island in the Channel Islands National Park. Authorities are searching the islands’ shorelines for survivors, the Coast Guard said.

As firefighters worked to put out the blaze, the boat sank in 64 feet of water, Rochester said. The hull is inverted.

The fire began about 3:14 am, Bemis said, and the Ventura County Fire Department said it was on the scene within 15 minutes.

Conception is based out of Santa Barbara Harbor, according to harbor patrol officer Nathan Alldredge.

Authorities responded to a call for fire and rescue support in the Platts Harbor area of Santa Cruz Island before 4 am, he said. When they arrived, they found “at least 30 passengers on board and (the boat was) fully involved in flames,” Alldredge said.

According to Truth Aquatics website, which conducts boat charters, Conception left Saturday on a three-day cruise and was scheduled to return Monday afternoon. CNN has reached out to Truth Aquatics for comment.