Investigators have begun interviewing witnesses for a preliminary report on the deadly dive boat fire off the Southern California coast early Monday.

Jennifer Homendy of the National Transportation Safety Board (NTSB) said Wednesday that the captain of the Conception, the boat’s owner, and other crew members underwent hours of detailed questioning. She declined to disclose details.

The captain and four crew members were the only survivors of the pre-dawn Labor Day fire off the Channel Islands that killed 34 people, whom authorities say were trapped below deck by the fast-spreading blaze.

Homendy said none of the survivors tested positive for alcohol, and drug test results were pending.

The boat’s fire protection systems also will be checked. The Coast Guard said the vessel wasn’t required to have sprinklers.

Fox 11 reported that the NTSB will have a preliminary report on the investigation in 10 days, which likely will include factual information but not a probable cause.

Only one person remained missing Wednesday after searchers recovered 33 bodies from the charred underwater wreckage.

The victims included an engineer for Apple who went on the trip with his wife and daughter to celebrate the teen’s 17th birthday, a special effects designer for Disney, a nature photographer, a nurse and a physics teacher from Northern California who was with his 26-year-old daughter.

FIRE BLOCKS ESCAPE FOR 34 BOAT OFF SOUTHERN CALIFORNIA COAST, INVESTIGATORS SAY

Those aboard also included students from Pacific Collegiate School in Santa Cruz, a public charter school for grades 7 through 12, the school said.

The only crew member to die was 26-year-old Allie Kurtz, who had quit her corporate job at Paramount Pictures to work on dive boats and recently had been promoted to deckhand on the Conception.

Flames moved quickly through the 75-foot vessel, blocking a narrow stairway and an escape hatch leading to the upper decks, investigators said.

DNA will be needed to identify the victims. Investigators will use the same rapid analysis tool that identified victims of the deadly wildfire that devastated the Northern California town of Paradise last year, Santa Barbara County Sheriff Bill Brown said.

The fire started shortly after 3 a.m. Monday as the boat sat anchored in Platt’s Harbor off Santa Cruz Island, among the rugged, wind-swept isles that form Channel Islands National Park.

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Coast Guard records show fire safety violations on the Conception in 2014 and 2016 were quickly fixed. There were no deficiencies found in February or August 2018 inspections.

While authorities have said they view the disaster as an accident, prosecutors from the U.S. attorney's office in Los Angeles and the Santa Barbara County district attorney's office are taking part in the investigation.

The Associated Press contributed to this report.