A Coast Guard spokeswoman, Capt. Monica Rochester, said during a news conference late Monday that the search would continue through the night. But she said the crew members were the only known survivors from the Conception. “It’s a very tragic event and we will search all the way through the night, into the morning. But I think we should all be prepared to move into the worst outcome,” she said.

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Passengers on the Conception slept in a single room tightly packed with bunk beds below deck, according to a floor plan of the boat on the website of Truth Aquatics, the company that operated the vessel and is based in Santa Barbara. Narrow staircases from the sleeping quarters and showers led to the galley.

It remains unclear what started the fire. But Mr. Brown said “you couldn’t have asked for a worse situation.”

“You have a vessel in the open sea that is in the middle of the night,” he said. “The sleeping compartment was on the bottom deck of the ship. They would have been sound asleep when this fire started.”

It was too early on Monday to say whether negligence had played any role in the fire. “The vessel has been in full compliance,” Capt. Rochester said at a news conference. The boat was equipped with a fire suppression system in the engine room. The vessel also had an onboard, built-in barbecue, according to the website.