(CNN) The Conception dive boat, which sunk on Labor Day killing 34 people off the coast of California, did not have a crewmember on roving overnight watch as required by its certificate, according to the National Transportation Safety Board's preliminary report and NTSB Chairman Robert Sumwalt.

"A crewmember sleeping in the wheelhouse berths was awakened by a noise and got up to investigate," the report says. "He saw a fire at the aft end of the sun deck, rising up from the salon compartment below. The crewmember alerted the crew behind the wheelhouse. As crewmembers awoke, the captain radioed a distress message to the Coast Guard."

Sumwalt told CNN in an interview that the incident is a "horrible, horrible tragedy," calling it the deadliest marine accident in nearly 70 years and the "the most deadly transportation accident that we've seen in a decade."

"Part of the certificate for this vessel required that there be constantly a roving watch person to keep an eye on the safety of the vessel," Sumwalt said. "The interviews, to this point, have indicated that the five surviving crew members were in fact asleep at the time that the fire broke out."

He said investigators have no way of knowing whether an additional crew member and any of the passengers, who were all below deck and died, were awake.

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