Divers with the San Luis Obispo County Sheriff's Dive Team prepare to search for a second day for missing people following a dive boat fire off Southern California's coast that killed dozens sleeping below deck, in Santa Barbara, Calif., Tuesday, Sept. 3, 2019.   (AP Photo by Christian Monterrosa )

SANTA BARBARA, Calif. (AP) — 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. Authorities have interviewed them but haven't said what efforts they made to help the 34 people trapped aboard before abandoning ship.

Twenty bodies have been pulled from the sunken vessel that had been anchored close to the shore of Santa Cruz Island, about 20 miles (32 kilometers) 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.

"We're sensitive to the fact that families have gathered today, some from outside of the area, to bring their loved ones home," Santa Barbara County Fire Chief Mark Hartwig told reporters. Those families "will rely on us to do everything in our power to find out happened aboard that vessel in the last moments of these family members' lives. That's our commitment."

DNA will be needed to identify all the victims, and authorities will be using the same rapid analysis tool that identified victims of the deadly wildfire that devastated the Northern California town of Paradise last year, the sheriff said.

Authorities have not released any victims' names, but Brown said he had heard anecdotally that they ranged from teenagers to people in their 60s. He said most appear to have been from Northern California, including Santa Cruz, San Jose and the San Francisco Bay Area.

Susana Rosas of Stockton, California, posted on social media that her three daughters, their father and stepmother were on board.

Kristy Finstad, 41, co-owner of Worldwide Diving Adventures, which chartered the boat, was identified in a Facebook post by her brother, Brett Harmeling of Houston who asked people to pray for her.

Pacific Collegiate School in Santa Cruz said some of its students and their parents were on the boat.

The Coast Guard called off its rescue operation Tuesday after determining there were no more survivors, Capt. Monica Rochester said.

The 75-foot (23-meter) Conception was on the last day of a three-day excursion to the chain of rugged, wind-swept isles that form Channel Islands National Park in the Pacific Ocean west of Los Angeles. The fire broke out shortly after 3 a.m. Monday as the boat sat anchored in Platt's Harbor off Santa Cruz Island.

The sheriff said all 34 victims were trapped on the boat's lower deck when flames quickly blocked both a narrow stairway and an escape hatch leading to the upper decks.

"The one crew member who was not rescued appears to have been asleep in the sleeping area down below with the passengers," Brown said.

It appears the crew sent multiple mayday calls, including one from the Conception in which a crew member could be heard saying he couldn't breathe.

A later one came from the fishing vessel The Grape Escape after the crew reached it in their small boat. One of the vessel's owners said he heard explosions as he watched the Conception go up in flames from about 100 yards away. Brown said the blasts could have been exploding scuba tanks or propane tanks.

Investigators have received written accounts from the five survivors and would interview them, Brown said. The National Transportation Safety Board, which also is investigating, said they were cooperating.

"I am 100 percent confident that our investigators will determine the cause of this fire, why it occurred, how it occurred and what is needed to prevent it from happening again," NTSB member Jennifer Homendy told reporters.

The Conception, based in Santa Barbara Harbor, was owned by Santa Barbara-based Truth Aquatics, founded in 1974. A memorial at the harbor has quickly grown as mourners come to pay their respects.

Actor Rob Lowe tweeted that he had been aboard the vessel many times. Dave Reid, who runs an underwater camera manufacturing business with his wife, Terry Schuller, also has traveled on the Conception and two other boats in Truth Aquatics' fleet and said he considered all three among the best and safest.

Schuller said the company's crews have always been meticulous in going over safety instructions at the beginning of every trip she's been on.

"They tell you where the life jackets are, how to put them on ... the exits, where the fire extinguishers are, on every single trip," Schuller said.

Coast Guard records show the boat's owners quickly addressed all safety violations from the last five years.

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Associated Press writers John Antczak and John Rogers in Los Angeles, Julie Watson of San Diego and Janie Har from San Francisco contributed to this story.

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This story has been corrected to show that a quote from Santa Barbara County Fire Chief Mark Hartwig was attributed to Sheriff Bill Brown.

James Miranda, right, of Santa Barbara, holds flowers and takes a moment to reflect at a dock near the Sea Landing at Santa Barbara Harbor in Santa Barbara, Calif., Monday, Sept. 2, 2019. A fire raged through a boat carrying recreational scuba divers anchored near an island off the Southern California coast early Monday, leaving multiple people dead and hope diminishing that any of the more than two dozen people still missing would be found alive. (AP Photo/Ringo H.W. Chiu)
Divers with the San Luis Obispo County Sheriff's Dive Team prepare to search for missing people following a dive boat fire off Southern California's coast that killed dozens sleeping below deck, on Tuesday, Sept. 3, 2019, in Santa Barbara, Calif. (AP Photo by Christian Monterrosa )
The fire broke out about 3 a.m. as the Conception was anchored off Santa Cruz Island, about 90 miles (145 kilometers) northwest of Los Angeles.; 2c; with ; ETA 10 a.m. ;
A photo left at a memorial for the victims of the Conception vessel reads
Divers with the San Luis Obispo County Sheriff's Dive Team prepare for a second day of searching for those still missing in Monday's fire on a dive boat, on Tuesday, Sept. 3, 2019 in Santa Barbara, Calif. A fire raged through a boat carrying recreational scuba divers anchored near an island off the Southern California Coast on Monday, Sept. 2, leaving multiple people dead and little hope that any of the people still missing would be found alive. (AP Photo by Christian Monterrosa )
Local residents with their dogs pause at a memorial for the victims of the Conception vessel outside of the Sea Landing at Santa Barbara Harbor in Santa Barbara, Calif., Tuesday, Sept. 3, 2019. Officials say no one likely escaped the flames that tore through a boat packed with scuba divers and the search for survivors has been called off. (AP Photo/Christian Monterrosa)
This Monday, Sept. 2, 2019, image from video released by the U.S. Coast Guard shows a Coast Guard Sector San Diego MH-60 Jayhawk helicopter video screen, as crew responds to the vessel
Local law enforcement and coroners work to retrieve the bodies found by diving teams after a deadly fire on the boat Conception, Monday, Sept. 2, 2019, in Santa Barbara, Calif. A fire raged through a boat carrying recreational scuba divers anchored near an island off the Southern California coast early Monday, leaving multiple people dead and hope diminishing that any of the remaining people still missing would be found alive. (AP Photo/Christian Monterrosa)
Local law enforcement along with search and rescue teams help unload the bodies of those who died in a diving boat fire, Monday, Sept. 2, 2019, in Santa Barbara, Calif. A fire raged through a boat carrying recreational scuba divers anchored near an island off the Southern California coast early Monday, leaving multiple people dead and hope diminishing that any of the remaining people still missing would be found alive. (AP Photo/Christian Monterrosa)
Local fire officials along with search and rescue teams walk off the dock after unloading the bodies of those who died in a diving boat fire, Monday, Sept. 2, 2019, in Santa Barbara, Calif. A fire raged through a boat carrying recreational scuba divers anchored near an island off the Southern California coast early Monday, leaving multiple people dead and hope diminishing that any of the remaining people still missing would be found alive. (AP Photo/Christian Monterrosa)
Coast Guard Captain Monica Rochester, left, and Santa Barbara County Sheriff Bill Brown address the media outside the Santa Barbara County Sheriffs headquarters in Santa Barbara, Calif., Tuesday, Sept. 3, 2019. Officials say no one likely escaped the flames that tore through a boat packed with scuba divers and the search for survivors has been called off. Authorities said Tuesday that no one has been found alive. Only five of crew members sleeping on the top deck were able to escape early Monday.  (AP Photo/Christian Monterrosa)
Maya Upton, of Santa Barbara, Calif., places flowers at outside of the Sea Landing at Santa Barbara Harbor in Santa Barbara on Monday, Sept. 2, 2019. (AP Photo/Ringo H.W. Chiu)
National Transportation Safety Board (NTSB) member Jennifer Homendi pauses as she speaks at a news conference in Santa Barbara, Calif., Tuesday, Sept. 3, 2019. Homendi discussed the ongoing investigation of how the Conception diving boat became engulfed in flames and killed dozens on board. (AP Photo/Christian Monterrosa)