SANTA BARBARA, Calif. — Flames roared through a boat of sleeping scuba divers so quickly that it appears none of the 34 people below deck could escape, authorities said Tuesday as they ended their search without finding anyone who was missing still alive from the Labor Day tragedy off the Southern California coast.

It’s not known what started the fire early Monday aboard the Conception, which carried scuba diving enthusiasts on a three-day excursion. It spread rapidly and flames blocked both exits out of the lower deck, where passengers and one crew member were sleeping in tight quarters, Santa Barbara County Sheriff Bill Brown said.

The captain and four crew members awake on the upper decks jumped off the front of the vessel, swam to an inflatable boat at the back and steered it to a ship anchored nearby.

Twenty bodies have been pulled from the sunken vessel that had been anchored close to the shore of Santa Cruz Island, about 20 miles off the coast and northwest of Los Angeles. Four to six other bodies have been spotted underwater, and divers are looking for the remaining people who are missing.

Among those believed to be dead are five members of one family and the marine biologist leading the diving tour.