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(CNN) —  

The 34 people feared dead in a California dive boat fire likely got trapped when the roaring blaze blocked their escape routes and confined them to the lower sleeping deck, authorities said.

Thirty-three bodies have been recovered after the fire consumed the 75-foot dive boat Conception off the Southern California coast early on Labor Day, leaving one presumed body yet to be found, the Santa Barbara County Sheriff’s Office’s joint information center said Wednesday.

Authorities have said 33 passengers and a crew member were believed to be in the below-deck bunkroom when the fire broke out shortly after 3 a.m. (6 a.m. ET) Monday, and that those 34 did not survive.

Five crew members who were elsewhere on the vessel – including the captain – jumped off the fully engulfed boat and survived, the Coast Guard has said.

Passengers were not locked in the sleeping deck, but they were unable to flee the burning boat, Santa Barbara County Sheriff Bill Brown said Tuesday.

“There was a stairwell to get down the main entryway, up and down, and there was an escape hatch. And it would appear as though both of those were blocked by fire,” he said.

The boat was about 20 miles off the mainland coast, near Santa Cruz Island in the Channel Islands National Park.

The cause of the fire is under investigation

The National Transportation Safety Board is investigating the fire. Adam Tucker, the NTSB investigator in charge, said the Conception was not required to have a black box on board and was not voluntarily fitted with one.

Coast Guard Capt. Jason Neubauer told reporters on Wednesday that the Conception was not required to have a sprinkler system.

Alcohol testing was done on four out of five surviving crew members, Neubauer said. The results were negative, officials said. One crew member was not tested as he was being transported to the hospital during that time frame. Drug test results are pending.

The crew is being cooperative with investigators, said NTSB board member Jennifer Homendy.

A mayday call revealed the confusion between a Coast Guard dispatcher and the Conception’s captain. But only the dispatcher’s words could be heard.

The captain apparently reports a fire and provides a location. The dispatcher is heard saying, “And there’s 33 people on board the vessel that’s on fire, they can’t get off? … Roger, are they locked inside the boat? … Roger, can you get back on board and unlock the boat, unlock the door so they can get off? … Roger, you don’t have any firefighting gear at all? No fire extinguishers or anything?”

Coast Guard Capt. Monica Rochester said there was “a lot of adrenaline, a lot of confusion” over the radio communication system. She said she believes the radio dispatcher “was trying to ask for information.”

She said “there are no locked doors in accommodation spaces” where passengers slept on the boat.

“The only privacy that you have … are curtains,” she said.

Captain made two mayday calls, company owner says

Truth Aquatics owner and operator Glen Fritzler spoke Tuesday to CNN affiliate Spectrum News 1, saying he’d never seen anything like the fire during the years he’s been in business.

“I’m numb,” he said. “It’s a complete tragedy. It’s horrible.”

The two entrances into the bunking area of the Conception were blocked by fire and there were no locked doors, Fritzler told the network.

Two mayday calls were made in the Conception’s wheelhouse, he said, and it’s during the second call that the captain is heard saying, “I can’t breathe.” The captain was the last to leave the boat, Fritzler said.

The crew members and captain feel horrible, he told Spectrum News 1.

“They’re a wreck. They’re an emotional wreck,” he said.

Truth Aquatics will shut down its operations for a couple weeks out of respect for the families involved, Fritzler said.

Rapid DNA technology to be used to ID victims

Thirty-three people had signed up to spend the Labor day weekend scuba diving and exploring colorful underwater sea life. Now their families are facing a long, agonizing wait for answers after the ship caught fire off Santa Cruz Island on the last part of the three-day trip.

Authorities will use rapid DNA technology to identify those whose bodies were burned beyond recognition, Brown said. DNA samples are being collected from family members.

The technology can simultaneously analyze five DNA samples in 90 minutes, and was used to quickly identify victims of the devastating Camp Fire in Northern California last year, federal authorities said. It can take weeks to get results in a traditional forensic lab.

The blaze swallowed most of the boat within minutes. Ventura County firefighters reached the boat within 15 minutes, but by then, it was engulfed in flames.

Firefighters struggled to extinguish the fire because the flames kept flaring back up, likely because of the fuel on board, the Coast Guard said.

CNN’s Sarah Moon, Holly Yan, Stella Chan, Jason Hanna, Theresa Waldrop and Eliott C. McLaughlin contributed to this report.