The 75-foot (23-meter) Conception dive boat caught fire and sank with passengers trapped below deck by the roaring blaze
The 75-foot (23-meter) Conception dive boat caught fire and sank with passengers trapped below deck by the roaring blaze Santa Barbara County Fire Department / Mike ELIASON

All six crew members were asleep when a fire broke out last week on board a California dive boat, leaving 34 people dead, the National Transportation Safety Board said Thursday.

In its preliminary report on the September 2 blaze -- the worst maritime disaster in modern California history -- the agency said "at the time of the fire, five crew members were asleep in berths behind the wheelhouse, and one crew member was asleep in the bunkroom."

Under federal law, the 75-foot Conception was required to have a night watchman who was awake and able to alert others to a fire or other mishaps, NTSB board member Jennifer Homendy told the Los Angeles Times.

The preliminary report said one of the crew members was able to raise the alarm after being awakened by noise around 3:15 am and seeing the fire.

"As crew members awoke, the captain radioed a distress message to the Coast Guard," the report said.

It added that five crew members frantically tried to reach the 33 passengers and one fellow crew member sleeping in the lower deck but jumped overboard after they were unable to open a forward window and were overwhelmed by smoke.

Two crew members and the captain then swam to the stern and reboarded the vessel but were unable to access the 34 victims.

The fire was so intense that firefighters were unable to board the vessel which sank as they tried to extinguish the blaze.

The boat had been on a three-day diving excursion around the Channel Islands, off the coast of Santa Barbara in southern California.

The blaze broke out on the last day of the trip, as the boat was anchored off Santa Cruz Island.

The cause of the fire is still unknown and initial interviews of some of the crew members revealed no mechanical or electrical issues onboard the vessel, the NTSB report said.

It said salvage operations are underway to recover the wreckage for examination and that the probe will look into regulations for such vessels as well as warning systems, evacuation routes and training.