The charred remains sank below the surface of the Roche Harbor Resort marina mid-evening Wednesday, after an 85-foot luxury yacht was destroyed by a vigorous fire that broke out in an interior cabin at the front end earlier in the day.

The boat is believed to have been carrying 1,600 gallons of diesel in its fuel tanks at the time, according to state Ecology officials.

Chief Steve Marler of San Juan Island Fire Department said the location of the fire and its intensity, and the fact that the yacht had been tied-off to the dock at the rear, all contributed to the difficulty of battling the blaze.

He said the department took an “extraordinary” step of sending fire fighters onto a burning boat in the early stages of combating the fire, but that they had to retreat and back out as flames and the intensity of the fire swelled.

“It was a problem to get there with fire hoses and a fire crew,” Marler said. “Anytime you’ve got a boat fire it’s going to be a real circuitous route to get to it. By policy we don’t go aboard a burning boat, we don’t train for that, but it seemed reasonable given the risks versus potential rewards early on.”

Marler said fire fighters arrived at the scene shortly after the department received an emergency call at 10:15 a.m. Five hours later, at about 3:10 p.m., he said the fire finally fizzled out as the bow of the boat dropped below the water. What remained of the yacht, valued at $4.5 million, sank at about 6 p.m.

The yacht was moored at the marina and advertised for sale by the Seattle office of Ocean Marine, a Taiwan-based manufacturer of Ocean Alexander megayachts and motoryachts. The vessel was unoccupied at the time, and the cause of the fire has yet to be determined, Marler said.

“Our greatest hope of finding a cause will be to know who was on the boat last and what was happening at the time,” he said. “My guess is that we really may never know for sure, but we might be able to figure out a plausible set of circumstances.”

Officials with the U.S. Coast Guard and the state Department of Ecology were at the scene to guide pollution prevention efforts throughout the afternoon and into the evening.

Employees of the resort’s marina encircled the burning boat with an oil-spill containment boom shortly after the fire broke out. Much of the area inside the boom was littered with smoke and fired-charred debris after the yacht sank. Nine members of Islands Oil-Spill Association were at the scene of the fire and were joined by another six to assist in the clean up early Wednesday.

Divers with Global Diving and Salvage emptied 500 gallons of diesel from the fuel tanks of the submerged wreckage late Wednesday, according to Ecology’s Dustin Terpening.

According to Terpening, a crane is expected to be brought to the site of the sunken boat by Thursday, as part of a plan developed by the Coast Guard, Global Salvage and the insurer of the sunken yacht, and the wreckage lifted from the water sometime Friday morning.

“Once the boat is raised they’ll be able to have a better look at how much fuel might still be onboard,” he said.