SAN DIEGO >> Four days after the initial spark, the fire on a U.S. Navy warship was still burning as firefighting sailors today inched their way deeper into its compartments in a painstaking search to find every smoldering hot spot.

Experts say the stubborn fire on board the USS Bonhomme Richard illustrates how difficult ship blazes are to put out once they tear through a vessel.

There have been pockets of fire throughout the 840-foot amphibious assault ship that have flared since it began Sunday morning. The fire was reported first in its lower area where armored vehicles are parked and where heavy-duty cardboard boxes, rags and other maintenance supplies were being stored. It 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, threatening to soften its steel.

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

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 in Jacksonville, Florida, for eight days.

Adding to the difficulty of the Bonhomme Richard fire was the fact that it had been undergoing maintenance since 2018, so there was scaffolding and other equipment and debris in the way. That also got in the way of firefighters. One of the ship’s fire suppression systems also was deactivated because of the maintenance project.

As of early Wednesday, helicopters had dumped 1,500 buckets of water on the ship, cooling the superstructure and flight deck to enable crews to move further inside the vessel and identify hot spots. They were battling two areas at the bow and the stern, Navy officials said. The fire was about 85% contained.

Retired Capt. Lawrence Brennan, a professor of international maritime law at Fordham University in New York, said even spraying water on a ship fire can be risky: If any aluminum on board had melted on plywood the combination could create aluminum carbide, which, in turn, can generate a flammable methane when sprayed with water.

“An uncontrollable fire like this one is among sailors’ worst fears,” he said, adding that’s why ships are designed to have so many compartments that can be closed off quickly with airtight doors.

Rear Adm. Philip Sobeck said the Bonhomme Richard’s design — which is like a mini-aircraft carrier — may have helped spread the blaze.

“For this class of ship, the open area above the vehicle storage is all open, a big hangar,” he said. “Once the fire hit that amount of oxygen, it found other ways to go up.”

He has said the full extent of the damage won’t be known until the fire is completely out and crews can access all the areas once it is safe to enter.

Sobeck, who is commander of the strike group that includes the Bonhomme Richard as its flagship, has said he is hopeful the ship can still be repaired but no one will know until that assessment is done.

He said four engineering spaces did not suffer major damage as initially feared, and the external structure of the ship is safe. Tugboats firing water cannons have kept the hull cool so it does not rupture and cause the one million gallons of fuel on board to spill in the bay.

Fuel was stored below the waterline and the risk of it spilling or exploding was now “very low,” Sobeck said, although the U.S. Coast Guard was standing by in order to place a boom to prevent the spread of oil should that occur.

It could cost an estimated $4 billion to replace the ship if it is deemed un-salvageable. The Bonhomme Richard had been docked since 2018 and was nearing the end of an estimated $250 million upgrade so it could start being used to deploy the Marine Corps’ F-35Bs in the Pacific.

More than 60 sailors and civilians have been treated for minor injuries, heat exhaustion and smoke inhalation.