OXNARD, California—Thirty-four people are missing after an inferno ripped through a 75-foot dive boat near Santa Cruz Island off the coast of Southern California early Monday.

The fire broke out on the Conception, owned by Truth Aquatics, around 3:00 a.m. PST, while it was anchored overnight only about 20 yards off Platts Harbor, officials said. Firefighters were still trying to douse the flames when the vessel sank in 60-foot waters.

Five crew members were awake and on the bridge when the flames began to consume the wooden-hulled vessel and they jumped off, officials said. They were rescued by the pleasure boat Grape Escape—but as the hours passed, the worst was feared for the others who apparently were trapped.

“The word I have received, is that they were below decks asleep,” U.S. Coast Guard Capt. Monica Rochester said at a press conference. “Right now they are conducting shoreline searches for any available survivors.”

Boats and helicopters were surveying the water, but more than 12 hours after the fire, the only known survivors were the crew members.

Bob Hansen, the owner of Grape Escape, told The Daily Beast that he and his wife were out on the water for the weekend, anchored in a cove on Santa Cruz Island, when they heard pounding on their boat.

“I put on some shorts and walk outside and open the door and here’s five guys in a little rubber boat and a 75-foot commercial boat on fire,” said Hansen, 73, from Madera, California. “It was really on fire... the whole thing engulfed in flames.

“The flames were shooting up 25 feet. I felt so helpless. It’s just burning. There were five tanks that were blowing up—or we thought there were—these big pops.”

Hansen brought the five crew members on board; one had a badly broken leg. He radioed the Coast Guard and waited for them to arrive, watching the scene unfold in horror, before ferrying the crew back to land.

“You can only imagine the tragedy. It’s horrendous,” Hansen said. “There are some things you can’t unsee.”

The Coast Guard struggled to put out the fire.

“It keeps being extinguished and re-flashing,” Coast Guard Senior Chief Aaron Bemis told CNN. Several hours later, the burning hulk sank, leaving its bow pointing out of the water.

The names of the passengers were not immediately released, and their relatives were anxiously awaiting news.

Shayna Kreps, who lives in the Bay Area, told The Daily Beast one of her family members was on the trip. She said her family learned of the disaster on the news and had not been able to reach authorities.

“We're angry that we’re unable to get any information,” she said.

The cause of the blaze was unknown, but because it was a diving trip, there could have been air tanks on board exploding and reigniting the flames. The FBI was on the scene and the National Transportation Safety Board said it was sending a crew to the site.

The Coast Guard said the vessel was “in complete compliance” and that the owner was cooperating with the operation and investigation.

Truth Aquatics’ fleet is custom-designed for diving, according to its website, which says the company “caters to divers or ocean enthusiasts seeking a stress-free getaway.” 

A calendar of events on the website indicated a three-day $665 Labor Day weekend trip was scheduled and being run by Worldwide Diving Adventures.

“Divers have the unique opportunity to explore the pinnacles of San Miguel Island,” World Diving wrote of the trip. “The beginning of September is the best time to be at San Mig, which see strong winds and swell during much of the year.”

Efforts to reach Truth Aquatics and Worldwide Diving were unsuccessful. But the founder of Truth Aquatics, who has been retired for years, was stunned by the tragedy.

“I spent 40 years running boats and we never had a fire... I'm just flabbergasted,” Roy Hauser told The Daily Beast, adding that he commissioned the Conception in 1980 and that it had undergone “tremendous remodeling” in the last three years.

Hauser wondered whether something electrical could have sparked the blaze given that it happened in the dead of night.

“The galley wouldn't be open at that hour... The generator is running all the time. There'd be no cooking activity at 4 o’clock in the morning. The crew only starts getting everything ready at 5:30 in the morning.”

Specifications for the vessel on the Truth Aquatics website showed it can accommodate 46 people—13 in double bunks and 20 in single bunks.