The fire on the USS Bonhomme Richard in San Diego Harbor has been extinguished, ending one of the worst infernos to rip through a U.S. warship outside of combat in recent years, the Navy announced Thursday.

“All known fires have been extinguished aboard USS Bonhomme Richard,” Rear Adm. Philip E. Sobeck said in a statement.

Teams were continuing to check every space to make sure no fire remained and until that process was complete an official investigation into the cause of the blaze that started Sunday would not begin, he said.

“We did not know the origin of the fire. We do not know the extent of the damage. It is too early to make any predictions or promises of what the future of the ship will be,” said Sobeck, commander of Expeditionary Strike Group 3.

The news came after the massive ship shifted during the night and listed toward the pier, prompting the Navy to pull off firefighting sailors.

The withdrawal of the roughly 30 sailors on board late Wednesday was out of an abundance of caution and there is no fear of the 840-foot (255-meter) vessel capsizing, said Lt. Cmdr. Patricia Kreuzberger. They were back on the ship within an hour.

The Navy was keeping a close eye on any movements as the ship settles after burning. The fire started in its lower armored vehicle storage area and quickly spread throughout the amphibious assault ship that is akin to a mini-aircraft carrier.

Helicopters dumped more than 1,500 buckets of water on the ship, which had been docked in San Diego harbor undergoing maintenance.

The Navy believes a spark from an unknown source first ignited heavy-duty cardboard boxes, rags and other maintenance supplies that were being stored in the lower vehicle storage area.

The fire traveled upward to the well deck — a wide hangar type area — and took off from there, Navy officials have said.

The fire at one point reached up to 1,000 degrees Fahrenheit (538 degrees Celsius), threatening to soften steel.

Experts said shipboard fires are difficult to douse.

“It’s very difficult to choke off oxygen in open deck spaces” and then to follow the flames into all the nooks on a craft, said maritime lawyer Rod Sullivan, who served in the Navy.

It’s not uncommon for ship fires to take days to extinguish, he added, pointing to a fire last month on a car-carrying cargo ship that burned for eight days in Jacksonville, Florida.

The difficulty was compounded aboard the Bonhomme Richard because it was undergoing maintenance and there was scaffolding, along with other equipment and debris in the way of firefighters. One of the ship’s fire suppression systems also was deactivated because of the maintenance project.

It could cost an estimated $4 billion to replace the ship if it is deemed un-salvageable. The Bonhomme Richard was nearing the end of a two-year-long upgrade estimated to cost $250 million. It was being done so the ship could start being used to deploy the Marine Corps’ F-35Bs in the Pacific.

More than 60 sailors and civilians were treated for minor injuries, heat exhaustion and smoke inhalation since Sunday.

(AP)