PORTLAND, Maine — Two weeks after a massive fire destroyed much of a historic fort on Great Diamond Island, the city and the state fire marshal’s office are still investigating the cause of the blaze.

The Nov. 23 fire caused damage estimated at $5 million to $9 million to the former U.S. Army barracks at 18 McKinley Court. No one was injured.

The structure, listed on the National Register of Historic Places, was being renovated to become a hotel called The Inn at Diamond Cove.

Fire Chief Jerome LaMoria on Monday said it is unclear when the investigation would be complete, or what the findings might reveal.

The involvement of the fire marshal is routine in such a large fire. The investigation is complicated by the size of the building, the amount of destruction and the number of workers who have been involved in the renovation, LaMoria said.

But meanwhile, the Fire Department is making some changes.

For example, LaMoria said the fire underscores the need to increase recruitment and training for the volunteer firefighters who safeguard the city’s island communities. That’s also a recommendation made by a 523-page performance audit of the department earlier this year.

In addition, LaMoria said he hopes to keep one of Great Diamond Island’s two fire engines indoors, averting the need to drain the engine’s coolant during the winter. Because the engine was recently drained, it “took some time” before it could respond to the recent blaze.

“When a fire on the islands goes undetected, it can get to a massive point very quickly. That was the case two weeks ago,” he said. “Time is an enemy for us when we’re dealing with fire.”