Smoke rises from the USS Bonhomme Richard at Naval Base San Diego Sunday, July 12, 2020, in San Diego after an explosion and fire Sunday on board the ship at Naval Base San Diego. (AP Photo/Denis Poroy)

SAN DIEGO (AP) — Firefighters on land, in the air and on the water on Monday were still battling a blaze on a Navy ship that injured at least 21 people and sent smoke billowing over San Diego.

The fire began Sunday morning in a vehicle storage area aboard the USS Bonhomme Richard, according to Rear Adm. Philip Sobeck, who said 17 sailors and four others, including firefighters, were treated at hospitals for smoke inhalation or other minor injuries.

It was proving to be a stubborn blaze to put out, requiring helicopters dumping water from above, firefighting vessels shooting seawater and fire trucks pounding water through hoses from the dock. Sobeck told the San Diego Union-Tribune that there was no ordnance on board, and while the ship holds a million gallons of fuel, that is “well below” any heat source.

About 160 sailors and officers were on board when an explosion and flames sent up a huge plume of dark smoke from the 840-foot (255-meter) amphibious assault vessel, which has been docked at Naval Base San Diego and was undergoing routine maintenance. That's far fewer than the thousand typically on the ship when it’s on active duty, said Mike Raney, a spokesman for Naval Surface Force, U.S. Pacific Fleet.

Sobeck, commander of Expeditionary Strike Group 3, told the Union-Tribune that the Navy thinks the fire began somewhere in a lower cargo hold where marine equipment and vehicles are stored.

The fire was not a fueled by fuel oil, hazardous materials or electrical causes, Sobeck told the newspaper. It was fueled by paper, cloth, rags or other materials in a standard fire. He said he is not concerned about the air quality or toxicity around the fire.

The explosion was probably caused by a change in air pressure, he told the Union-Tribune.

All crew members were accounted for, said Admiral Mike Gilday, Chief of Naval Operations.

“We are grateful for the quick and immediate response of local, base, and shipboard firefighters aboard USS Bonhomme Richard,” Gilday said in a statement.

The 23-year-old ship has the capacity to deploy and land helicopters, smaller boats and amphibious vehicles. Because of its age, a fire could be particularly destructive, especially if it reached the engine room and other tight spaces with machinery, said Lawrence B. Brennan, a professor of admiralty and international maritime law at Fordham University in New York.

“The heat of a fire of this nature can warp the steel, and that can be a major problem for any ship,” said Brennan. “On an older ship, it's even more of a problem.”

Two other docked ships, USS Fitzgerald and USS Russell, were moved to berths away from the fire, officials said.