(CNN) -- Firefighters extinguished a blaze in a nuclear submarine early Thursday at a U.S. Navy shipyard in Maine after battling it for hours, according to a statement released by the shipyard.
Seven people were injured and were treated either on the scene or at "a local medical facility," the statement from the Portsmouth Naval Shipyard read. Among those injured were three firefighters from the shipyard, two ship crew members and two civilian firefighters, who assisted the Navy's fire crew.
The USS Miami's reactor was not operating at any time the fire broke out and remained unaffected and stable throughout, said Capt. Bryant Fuller, commander for the shipyard, which is in Kittery, Maine.
The sub was in dock at the shipyard when the blaze began, where it has been "since early March for an overhaul." Firefighters first responded to the fire at 5:41 p.m. ET Wednesday.
The fire started in a forward part of the ship away from the reactor, affecting "primarily living areas and command and control spaces," Fuller said.
"The nuclear propulsion spaces were physically isolated early in the event from the forward compartment fire," according to the statement.
The USS Miami is capable of carrying Tomahawk cruise missiles and Mark 48 torpedoes. No weapons were on board, Fuller said.
State, local and federal authorities were notified of the incident.
"The cause of the fire is not known at this time," Fuller said. "A full investigation will be conducted."
Portsmouth Naval Shipyard was established on June 12, 1800, and is the oldest continuously operating shipyard run by the Navy, according to Gary Hildreth, a public affairs officer for the facility.
CNN's Larry Shaughnessy and CNN's Chris Boyette contributed to this report.