SANTA BARBARA, Calif. — A 13-month investigation into a recreational dive boat that burned and sank over Labor Day weekend last year has failed to find a definitive cause of the deadly fire, which killed 34 people aboard.

During a virtual hearing Tuesday in Washington, investigators told the National Transportation Safety Board that the Conception, the 75-foot dive boat, was too damaged when recovered from the sea bed to pinpoint the source of the fire.

But interviews with survivors and previous passengers on the boat, along with comparisons to its sister dive boat, suggested that the fire began where people charged their cellphones — in the common salon area of the boat, a deck above where 33 passengers and one crew member slept and perished.

Those chargers had caused a fire on the Vision, the Conception’s sister vessel, on a previous trip. But that fire was quickly extinguished because it was detected early.

Investigators also found there were no smoke detectors in the salon; they were not required by federal regulations governing the operation of small commercial passenger vessels.

One staff recommendation is to require smoke detectors in common areas in the future; had such devices been present, they probably would have prompted an earlier evacuation of Conception passengers and saved lives.

Investigators also found that the Conception’s crew did not follow requirements for having someone work as an overnight roving patrol, another measure that could have led to a more rapid firefighting effort and evacuation. One investigator told the board this failure “directly led to the high number of casualties.”

A relatively novice crew — one member had been with the boat for just two months — was also poorly trained in fire safety, investigators found. The dive boat passengers were not given a required briefing on arrival and were left for hours to navigate the safety procedures through a “welcome aboard” card.

“I would say that complacency but also a lack of imagination by the crew and the Coast Guard played a part,” said Bruce Landsberg, the NTSB’s vice chairman. “No one could imagine this happening.”

The fire aboard the Conception, an institution here among recreational divers, was among the nation’s worst sea disasters in recent history.

It also was the second ­mass-casualty event in as many years in this city stretching between steep mountains and the sea, where nearly two dozen people died in devastating ­post-wildfire mudslides in January 2018. Hand-painted signs remembering the Conception victims appeared within days on doors, lamp posts and bulletin boards around the everybody-knows-everybody harbor, which is also a tourist destination.

Several crew members, including the captain, survived the fire. The decades-old company that ran the boat, which shuttled countless amateur divers in the region on their first scuba-certification dives to the wild Channel Islands, suspended its operations within weeks of the accident. Landsberg noted that Truth Aquatics had a good safety record and was “a solid company, unlike others we see.”

Truth Aquatics has since restarted its dive charters with two remaining boats, the Truth and the Vision. The company’s website still features a picture of its fleet that includes the boat that burned.

The Conception sank in the predawn hours of Sept. 2, 2019, as the 33 passengers aboard slept in the lower decks. The boat was anchored off the north coast of Santa Cruz Island, the largest of the protected Channel Islands that loom a little more than 25 miles off the mainland coast here.

From the start, the rescue and investigation proved a challenge. Coast Guard boats and helicopters rushed to the site. But the boat burned and sank quickly in dark water that, although just 60 feet offshore, was as deep as 65 feet.

Within days, divers and surface crews had recovered more than two dozen bodies, ranging in age from 17 to over 60. Those on board were from dive clubs in Santa Cruz and San Jose, cities several hundred miles north of here, participating in a charter for the long holiday weekend.

To identify the badly burned bodies, medical forensic teams used techniques developed after the 2018 wildfire that turned the town of Paradise, Calif., to ash and killed 85 people.

The divers had completed a night excursion when they retired to the lowest of the Conception’s three decks, where bunks were laid out, smoke detectors in each aisle. One crew member slept in the bunk area, too.

A crew member on the top deck noticed a glow coming from the sun deck at 2:35 a.m., and when he went to investigate, he found fire blocking his way to the second deck. Another crew member lowered himself to the second deck, finding the escape hatch and stairwells to the bunk area blocked by flames.

Several crew members abandoned ship in a skiff soon after and reached a nearby boat, called the Great Escape, where the captain made a distress call to the Coast Guard at 3:29 a.m. Scrambling to reach the remote site, the first Coast Guard vessel did not arrive until an hour later, once much of the top two decks had burned.

It took 10 days to pull the burned hull, precariously perched on the bottom, from the sea bed and take it to a naval station in Ventura, Calif., where investigators got to work. Almost nothing remained of the areas where the fire was believed to have started.

Board members appeared most concerned by the lack of an overnight watch, known as a “roving patrol.” Investigators said the regulation was difficult to enforce but would probably have helped save lives if implemented in this case.

One photograph presented during the hearing showed numerous cellphone charger cables draped over the back of salon seats on the boat. One of the five surviving crew members told the Coast Guard that sparks emanated from an outlet when he tried to plug in his cellphone that day, but NTSB investigators did not interview the crew member.

“There were clear and heartbreaking opportunities to break the chain of circumstances that led to this tragedy,” said Thomas B. Chapman, an NTSB member.