Fire ripped through a hotel under construction on Great Diamond Island in Casco Bay early Saturday, causing an estimated $5 million to $9 million in damage.

Portland Fire Department officials said the fire broke out about 4:25 a.m. at the Inn at Diamond Cove at 18 McKinley Court. The fire, which required much equipment to extinguish because it was so large, was under control by 10 a.m.

No one was injured in the blaze.

City firefighters left the island around 5 p.m. Saturday, Portland spokeswoman Sheila Hill-Christian said, but volunteers and island security guards planned to keep watch on the site overnight to make sure hot spots didn’t flare up.

Deputy Fire Chief Robert Wassick said the blaze was reported by a Casco Bay Bridge tender who spotted a large fire on the horizon in Casco Bay.

The department’s fireboat and 20 firefighters were sent to the scene. They arrived at the island about 4:50 a.m. to find the three-story brick structure engulfed in flames. The building’s roof collapsed during the fire.

The building was being developed by Diamond Cove LLC into a full-service hotel and condominium complex with a variety of room types at the Fort McKinley site, which is on the National Register of Historic Places, said Jeff Levine, the city’s planning and urban development director. The head of the development group, David Bateman, did not respond to a phone call and an email seeking comment Saturday.

Fort McKinley was built at the turn of the 20th century, designed to protect Portland Harbor during the Spanish-American War. It closed after World War II and was redeveloped in the 1990s into the upscale Diamond Cove community.

The building, the last of the unredeveloped structures on the site, was undergoing renovation and due to open as the Inn at Diamond Cove next year. Originally the hotel project was opposed by some homeowners, who went to court to try to block it.

The construction was nearing completion, Levine said.

Wassick said the state Fire Marshal’s Office will help with the investigation into the cause of the fire because of the scope of the damage. Sgt. Joel Davis of the Fire Marshal’s Office said it could take several days to make the building, which was a complete loss, safe for investigators.

City officials said investigators would likely interview witnesses Monday before beginning a thorough search of the debris.

The cause of the blaze has not been ruled suspicious at this point.

Twenty volunteer firefighters from Great Diamond Island’s station and Long Island also responded to the fire.

About 20 people live on Great Diamond Island in the winter.

Staff Writer Edward D. Murphy contributed to this report. 

Beth Quimby can be contacted at 791-6363 or at:

bquimby@pressherald.com