Investigators said a single mayday call came from the boat reporting the fire (Reuters)

A team of federal investigators on Tuesday promised an exhaustive probe into a fire that killed all 34 passengers on a dive boat, many of whose charred bodies were believed trapped in the sunken wreckage off the California coast.

After recovering 20 bodies from the 75-foot (23-metre) Conception or from the waters where the dive ship quickly sank off Santa Cruz Island, officials said they believed none of the 14 victims initially classified as missing had survived the fast-moving flames.

"There were several other victims that were seen by the divers - between four and six - that are still between the wreckage, but due to the position of the boat they were unable to be recovered before nightfall," Santa Barbara County Sheriff Bill Brown told a news conference.

"Today, efforts will be made to stabilise the boat so that divers can safely enter it, search it and recover additional victims," Brown said.

The five survivors, the boat captain and five members of his crew, were above deck when the blaze broke out at about 3:15 a.m. Pacific time and escaped in an inflatable boat. A crew member who perished was believed to have been sleeping below deck with the passengers.

National Transportation Safety Board member Jennifer Homendy said 16 investigators were already taking part in the investigation, including specialists in operations, engineering, survival factors and fire analysis.

She said the experts would collect all perishable evidence while on scene for an estimated seven to 10 days but that the Conception would remain on the ocean floor more than 60 feet below the surface until a site survey had been completed.

A few scant details about the victims, who ranged in age from 17 to 60, began to emerge as emergency workers planned to use DNA analysis https://www.reuters.com/article/us-california-fire-dna/california-investigators-to-use-war-zone-dna-analysis-technique-to-identify-boat-fire-victims-idUSKCN1VO2O2 to positively identify the remains of the 20 bodies recovered so far.

Most of the victims were from the Santa Cruz and San Jose area, authorities said.

"Terrifying"

A memorial to the victims grew alongside a dock not far from where the ship was usually docked in Santa Barbara as members of the close-knit boating community reeling from the tragedy wove flowers into a wooden fence and constructed a makeshift memorial https://www.reuters.com/article/us-california-fire-memorial/we-all-have-a-personal-connection-dockside-memorial-grows-for-california-boat-fire-victims-idUSKCN1VO2S8 with flowers and candles.

"It's just such a horrific notion to think what the people down in the below decks, the people sleeping down there must have gone through," said Judy Weisman, 72. "How terrifying."

An audio recording of a desperate call made to the U.S. Coast Guard as flames engulfed the boat offered a glimpse into that terror as a man could be heard pleading for help.

"Mayday, mayday, mayday!" he said in the garbled recording of the call.

"That's a distress, this is the Coast Guard Sector Los Angeles on channel 1-6, what is your position ... and number of persons on board? Over," the dispatcher answered.

"Twenty-nine. Twenty-nine POB," said, using the abbreviation for "people on board" a vessel. "I can't breathe! ... Twenty-nine POB."

The dispatcher requested the GPS location of the vessel at least two more times but the caller apparently fails to respond.

Kristy Finstad, 41, was leading the dive trip on the Conception, according to a Facebook post shared on Monday by her brother, Brett Harmeling.

Finstad, a scuba diving instructor and marine biologist, was one of the owners of Worldwide Diving Adventures, the firm that chartered the boat for a three-day excursion to the Channel Islands.

A sea shell inscribed with the name "Kristy" was left along the wooden fence at the dock.