KITTERY, Maine >>  Officials hoped to begin venting smoke and noxious fumes from a nuclear-powered submarine on Thursday so they could get inside to assess damage from an intense blaze that swept through the forward compartments.

Workers had to wait for the USS Miami to cool enough for fresh air to be safely introduced without risk of another fire.

The fire broke out Wednesday evening while the sub was in dry dock during a 20-month overhaul at Portsmouth Naval Shipyard. Authorities said nuclear components were not threatened, but firefighters had to work in cramped quarters and extreme heat.

Rear Adm. Richard Breckenridge praised the submarine’s crew, shipyard firefighters and additional firefighters from Maine and New Hampshire, saying there were “a lot of heroes that worked together to save the ship.”

“The fire spread to spaces within the submarine that were difficult to access, presenting a challenging situation for initial responders. But they persevered in incredible heat and smoke conditions, demonstrating exceptional courage,” the admiral told reporters at the entrance of the shipyard.

The fire damaged forward compartments including living quarters, the command and control center, and the torpedo room, leading to speculation whether costly repairs would be undertaken on the 22-year-old Los Angeles-class attack submarine.

“The duration of the fire suggests extensive damage that could render the vessel useless. These submarines were designed decades ago. So they’re no longer state of the art,” said Loren Thompson, defense analyst at the Lexington Institute. “If this vessel returns to service, I will be amazed.”

Working in the submarine’s favor was the fact that workers had removed some of the equipment and gutted part of the submarine during the retrofit, said U.S. Rep. Chellie Pingree after meeting with the shipyard commander.

There were no details on th ecause, and an investigation will take some time, said Breckenridge, commander of Submarine Group Two in Groton, Conn., where the USS Miami is based.

Firefighters from more than a dozen towns joined in battling the fire, first reported at 5:40 p.m. It wasn’t doused until early Thursday.

Pingree described it as a “hot scary mess.”

“It takes a lot of guts to into a burning building. But the idea of going into a submarine full of hot toxic smoke — that’s real courage,” she said.

Two sailors, three shipyard firefighters and two civilian firefighters were hurt, but their injuries were minor, officials said.

Firefighters isolated the flames so they would not spread to nuclear propulsion spaces at the rear of the submarine. There was nuclear fuel on board the sub, but the reactor has been shut down for two months and was unaffected.

Residents reported hearing sirens from fire trucks and ambulances throughout the night, and the smoke spread over the area.

“It smelled like plastic burning,” said Janet Howe of Kittery, who lives three-quarters-of-a-mile from the shipyard.

Reporters were not allowed onto the base to see the submarine Thursday. But Pingree and others who viewed the vessel said there were no outward signs of damage, because the fire was contained inside the 360-foot-long hull.

The rear compartments including the nuclear propulsion unit remained habitable, and crew members never left that part of the sub during the fire, Breckenridge said.

It was unclear how many people were aboard the vessel or what type of work was being done when the fire started. The submarine, commissioned in 1990, has a crew of 13 and 120 enlisted personnel. It arrived at the shipyard March 1.