Boats like the Conception, which caught fire and sank this month, are required to have a crew member keep watch at night

All six crew members were asleep when a fire broke out aboard the scuba diving boat Conception in southern California, killing 34 people trapped below deck, federal investigators announced on Thursday.

A preliminary two-page report by the National Transportation Safety Board (NTSB), which has been investigating the tragedy, said five crew members had been sleeping in their quarters behind the wheelhouse on the second deck and another below deck when the fire broke out. All but one survived the inferno; the cause of the blaze has yet to be determined.

Boats like the Conception, which caught fire around 3am on 2 September and sank off the coast of Santa Barbara, are required to have a crew member keep watch at night. Federal authorities are conducting a criminal investigation into the deadly fire and could bring charges under a statute known as seaman’s manslaughter.

Five crew members, including the captain, managed to escape by jumping overboard but dozens trapped in the boat’s bunkroom are believed to have died of smoke inhalation.

The disaster was the worst in California’s recent maritime history and sent shockwaves through the diving community and across the state. The remains of the final victim still unaccounted for were recovered on Wednesday.

The victims included a family of five, a girl celebrating her 17th birthday with her parents and a friend, and a 26-year-old crewmember who was thrilled by her recent promotion to deckhand. Others included the marine biologist who led the three-day tour and couples who shared a love of the water. So far nine victims have been officially identified, ranging in age from 25 to 62.

California boat fire: names of nine victims released at end of tragic week Read more

The report comes as investigators seek to determine the cause and try to recover the wreckage of the Conception from the bottom of the sea amid an ongoing criminal investigation conducted by the FBI, Coast Guard and the US attorney’s office.

The NTSB says one of the crewmembers on the upper deck awoke to a noise and saw flames rising from the middle deck. He alerted the rest of the crew as the captain issued a panicked mayday call to the Coast Guard.

The crew, finding the ladder to the main deck on fire, jumped down – with one breaking a leg in the process. They tried to reach the others through a window but couldn’t open it. They were forced to jump overboard when they became “overwhelmed by smoke”.

The captain and two crewmembers swam to the vessel’s stern and reboarded the boat, according to the report. They opened the engine room’s hatch but didn’t find any fire. With other access routes blocked, they launched the boat’s skiff, picked up the other two crewmembers and went to a nearby vessel.

Once onboard, the captain continued to send mayday calls as two crewmembers returned to the Conception to search for survivors near the burning wreckage.

There were none.

As crews work to recover the wreckage of the burned-out Conception from the bottom of the sea, the Coast Guard has issued additional safety recommendations in the wake of the tragedy, such as limiting the unsupervised charging of lithium-ion batteries and the use of power strips and extension cords.

The recommendations also suggest owners and operators of vessels review emergency duties with the crew, identify emergency escapes, check all firefighting and lifesaving equipment onboard, and look at the condition of passenger accommodation spaces for “unsafe practices or other hazardous arrangements”.

Coast Guard records show the Conception passed its two most recent inspections with no safety violations. Previous customers said the company that owns the vessel, Truth Aquatics, and the captains of its three boats, were very safety conscious.

Truth Aquatics filed a lawsuit last week in US district court under a pre-civil war provision of maritime law that allows it to limit its liability.