The Coast Guard has issued a safety bulletin following the California boat fire that killed 34 people, recommending that commercial boat operators limit unsupervised charging of cellphones and other electronic devices.

A brief, preliminary report on the Labor Day fire that destroyed the dive ship Conception near Santa Cruz Island was issued Thursday by the National Transportation Safety Board, but it did not address the cause of the fire. The report did note that three crew members said they knew of no mechanical or electrical issues with the boat.

The report said that all six crew members were asleep when the fire broke out. Boats like the Conception, which caught fire around 3 a.m., are required to have a crew member keep watch at night.

A lawyer representing Truth Aquatics, the company that owned the boat, disputed that part of the report.

“We do have witness testimony that seems to contradict the notion that the entire crew was asleep,'' attorney Douglas Schwartz said in a statement. "We do know that one crewmember checked on and around the galley area at around 2:30 a.m., approximately 30 minutes before the fire broke out.”

The Coast Guard said it has convened a Marine Board of Investigation to determine the cause of the blaze. The bulletin noted it does not have to wait for the board's findings before taking "immediate and positive" action.

“In some instances, our marine casualty boards identify pressing safety issues related to vessel stability, the engine room or lifesaving and firefighting equipment,” said Capt. Jason Neubauer, chair of the Marine Board of Investigation. “In those instances, we issue safety alerts or bulletins."

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The recommendations included ensuring that all required firefighting and safety equipment is on the boat and operational, that emergency escapes are clearly recognizable and functional, and that crew members understand their roles.

Boat operators also should "reduce potential fire hazards and consider limiting the unsupervised charging of lithium-ion batteries and extensive use of power strips and extension cords," the bulletin said.

"The intensity of the fire surprised people," Peter Goelz, former managing director of the National Transportation Safety Board, told USA TODAY. "If it was being fed by lithium batteries, that might explain it."

Goelz says he's never heard of charging stations being linked to a boat fire – but he was not surprised by the bulletin. On commercial airplanes, crew members have gloves, tongs and flame-smothering bags at the ready, he noted.

More than 30 divers spending a long weekend packed on a boat could have a lot of phones, cameras and laptops to charge, he said. One survivor even suggested the fire may have started in an area where electronics were charging.

"I've heard that a lot of attention is going there," Goelz told USA TODAY. "Did they have a charging station of epic proportions? Were electronics stacked up? We don't know yet."

Others issues being reviewed include passenger access to escape hatches.

The NTSB report said one of five crew members sleeping in the wheelhouse was awakened by a noise. He found a fire rising from a compartment below, awakened the other crew members and the captain radioed a distress message to the Coast Guard, the report said.

Crew members attempted to reach the bunk area where passengers and one crew member were sleeping but could not because of the flames, the report said. The captain and two others jumped into the water and swam around the boat and entered from another area but still could not access the passengers because of the fire.

They then launched a small skiff, picked up two crew members from the water, and went to a nearby recreational boat where they continued efforts to bring help.

"Local Coast Guard and fire departments arrived on scene to extinguish the fire and conduct search and rescue," the report said. "The vessel burned to the waterline by morning and subsequently sank in about 60 feet of water."

The Coast Guard Investigative Service, FBI and Bureau of Alcohol, Tobacco and Firearms are supporting a Department of Justice criminal investigation into the tragedy, the Coast Guard said.

Dan Salas, the CEO and owner of Harbor Breeze Cruises in Long Beach, Calif., told the Los Angeles Times that the Coast Guard closely scrutinized firefighting equipment and emergency access on his seven ships during annual safety inspections this week. He said he supports the Coast Guard efforts.

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A total of 39 people were aboard the boat for a holiday weekend expedition when the fire started. Five crew members who were on the deck fled and were rescued. The victims, 21 women and 13 men ranging in age from 16 to 62, apparently died of smoke inhalation, authorities have said.

Divers found the body of the last victim Wednesday. DNA testing was being conducted to confirm identities of seven of the 34 victims, the Santa Barbara County Sheriff's Office said.

Sheriff Bill Brown said the sleeping compartment was on the bottom deck of the ship and that the passengers likely were asleep when the fire started.

"This is probably the worst-case scenario you could possibly have," Brown said that day. “You have a vessel that’s on the open sea in the middle of the night. Fire is the scourge of any ship. ... You couldn’t ask for a worse situation.”

Contributing: The Associated Press