The US Coast Guard released a safety bulletin asking the owners and operators of commercial boats to think about limiting the unsupervised charging of cellphones and other electronics that use lithium-ion batteries.

The measure -- among others recommended in the bulletin -- comes after a fatal boat fire earlier this month sank the 75-foot dive boat Conception off the Southern California coast, killing 34 people. Five crew members survived.

The cause of the fire remains under investigation. National Transportation Safety Board Chariman Robert Sumwalt said Thursday the boat was required to have a roving watch person, but crew members were asleep.

The Coast Guard said its safety bulletin contained regulations that would serve as a reminder for the owners and operators of commercial vessels, related to fighting fires, saving lives and providing for escape in case of emergencies.

Among the recommendations is for operators is to "consider limiting the unsupervised charging of lithium-ion batteries and extensive use of power strips and extension cords."

Lithium-ion batteries are rechargeable batteries commonly used in electronics such as cellphones.

NTSB board member Jennifer Homendy previously said investigators were looking at the Conception's engineering and wiring, and were aware "there was a lot of gear on board with the cameras and the crew, cameras, phones, extra batteries. And chargers."

The safety bulletin also recommended boat operators review the crew's emergency duties, ensure logs on emergency drills and equipment maintenance are up to date and make sure emergency escapes are clearly identified. All required firefighting and lifesaving equipment should be operational, the bulletin said.

The Coast Guard also convened a four-member Marine Board of Investigation (MBI) to look into the factors that led to the incident, calling it the Coast Guard's "highest-level marine casualty investigation."

"A MBI is a lengthy and detailed process that may take a year or longer to complete," the Coast Guard's statement said. "It consists of witness interviews, public hearings, and evidence collection and analysis."

CNN's Paul Vercammen contributed to this report.

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