LOS ANGELES — Crew members on a dive boat say they were never instructed on emergency procedures before a predawn fire swept through the vessel as it was anchored off the Southern California coast, killing 34 people as they slept below deck, according to federal documents released Wednesday.

National Transportation Safety Board investigators say the cause of the blaze aboard the Conception remains undetermined but a possible ignition point was phones and other electronics plugged into outlets. One crew member told investigators he saw sparks when he plugged in his cellphone hours before the fire.

The boat was carrying 33 passengers on a Labor Day weekend scuba diving expedition last year. The fire broke out on the final night as the Conception was anchored off Santa Cruz Island, about 25 miles south of Santa Barbara, the boat’s home port.

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