All six crew members on a California dive boat that erupted in flames on Labor Day, killing 34 people, were sleeping when the fire broke out, the National Transportation Safety Board said on Thursday.

A U.S. Coast Guard spokesman has previously said that a safety certificate for the 75-foot Conception required that at least one crew member serve as a "night watchman" on the ship while passengers slept.

The disclosure came in a preliminary report issued by the NTSB, which says that crew members were not aware of any electrical or mechanical issues with the vessel before the blaze.

The NTSB said it was still trying to determine what sparked the flames.

The Conception caught fire at about 3:15 a.m. on Sept. 2 and sank off Santa Cruz Island. Only five crew members escaped. Recovery of the final body had been delayed by weather conditions that complicated dive operations.