PORT CLYDE, Maine — A fire investigator said Monday he won’t be able to determine the cause of a Christmas morning fire that destroyed a metal building used by the Monhegan Boat Co. for storage and as a workshop.

The fire reduced the Factory Road building and its contents to rubble and ashes, a short distance from the Monhegan boat dock.

“We’ve closed the investigation,” said Sgt. Ken Grimes of the State Fire Marshal’s Office.

Firefighters from St. George, Thomaston, South Thomaston and Owls Head responded to the alarm, and the Rockland Fire Department was on standby for those communities while their departments were fighting the fire, which was reported at 5:50 a.m. Thursday, according to St. George Fire Chief Tim Polky.

One firefighter, Steve Jarrett, was hospitalized for overexertion, Polky said.

The Monhegan Boat Co. operates the private ferry service to Monhegan Island.