A marine biologist and two high school students are among those believed to have died after a fire erupted in the middle of the night on a boat off the Southern California coast.

The US Coast Guard has suspended its search efforts after confirming that at least 25 people had died. Five of the six crew members who happened to be awake when the fire began escaped by jumping into an inflatable boat they then steered to a nearby vessel. The end of the search means the Coast Guard has presumed all 33 passengers and the one additional crew member died after the blaze.

Among those is thought to be Kristy Finstad, 41, who was on the boat to assist her family-owned scuba diving company as it embarked on a multi-day expedition, the Los Angeles Times reported on Tuesday. It is believed she was sleeping when the fire began at about 3.15am, officials said.

The two students on the ship who are now feared dead attended the elite Pacific Collegiate School in Santa Cruz. The school recorded a message on its answering machine that said: “Our hearts and prayers are with the victims and for the families of those missing, particularly those of our students.”

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The Conception, the boat involved in the fire, was owned by Truth Aquatics, a Santa Barbara-based company founded in 1974. The vessel was on the final day of a Labour Day weekend cruise when the fire erupted and a mayday call was made.

The boat sank to the bottom of the ocean floor nearly four hours after the fire began, with several victims still remaining underneath the wreckage, officials said. They were reportedly unable to retrieve all of the bodies under the boat due to weather conditions and other concerns.

"It is never an easy decision to suspend search efforts," Coast Guard Captain Monica Rochester said at a news conference. "We know that this is a very difficult time for families and friends of the victims."

“This is probably the worst-case scenario you could possibly have,” Santa Barbara County Sheriff Bill Brown said in a statement. “You have a vessel that’s on the open sea, that is in the middle of the night. Fire is the scourge of any ship.”

Coast Guard records show inspections of the Conception conducted last February and in August 2018 found no deficiencies.

Earlier inspections found some safety violations related to fire safety.

A 2016 inspection resulted in owners replacing the heat detector in the galley and one in 2014 cited a leaky fire hose.

Records show all safety violations from the last five years were quickly addressed by the boat’s owners.

It wasn’t immediately clear when the bodies on the ocean floor might be retrieved or when divers could search the boat for others. Officials have not yet identified the remains of those killed in the fire, and did not provide information about those who are still missing.

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Investigators have also not yet identified a cause for the fire.