Ingalls Shipbuilding is investigating a fire inside the superstructure of a $3 billion amphibious assault ship the Pascagoula shipyard is constructing for the U.S. Navy.

Ingalls spokesperson Kimberly Aguillard said the fire occurred last Thursday during the evening shift aboard the future USS Bougainville (LHA 9). Ingalls fire and safety personnel responded to the call and were able to keep the fire contained within a handful of compartments within the superstructure.

Two shipbuilders were transported to a local hospital for treatment of smoke inhalation and released later that same night. Four others were treated onsite and no additional injuries have been reported, Aguillard said.

The cause of the fire remains under investigation, with initial findings indicating there was “hot work” being performed in the area of the fire. Aguillard said the amount of smoke was “significant,” but mostly limited to the affected compartments.

A full review of the events, including a timeline, is “under development,” she said.

“It’s important to note that Ingalls Shipbuilding has a fully functional fire department and emergency responders on-site,” Aguillard said. “It is part of the shipyard’s commitment to ensuring we are prepared for any emergency. Our highly trained responders regularly and proactively support accident response to simulated conditions at the shipyard. This ensures there is a coordinated response by various shipyard departments and Navy personnel in case of an emergency.

Bougainville is being constructed at a cost of $3.1 billion. Fabrication began in October 2018 and her keel was authenticated in March 2019, with launch set for later this year. LHA 9 is due to be delivered to the Navy sometime in 2024.

Ingalls is the Navy’s sole builder of large-deck, amphibious warships for the Navy.