PORTLAND — More than two days after a blaze tore through a historic island fort, causing as much as $9 million in damage to a property in the process of becoming a luxury hotel, a state fire investigator said the building may not be stable enough for a full probe into the fire’s cause.

Sgt. Joel Davis of the state fire marshal’s office told the Bangor Daily News on Monday afternoon that state investigators spent the day interviewing Portland firefighters and employees about the blaze, which was reported at around 4:30 a.m. Saturday and gutted the 19th century Fort McKinley U.S. Army barracks on Great Diamond Island.

Portland Deputy City Manager Sheila Hill-Christian said Saturday that the fire caused roughly between $5 million and $9 million worth of damage to the property, which was in the process of being redeveloped as a destination hotel, the Inn at Diamond Cove.

David Bateman, one of the original developers behind the hotel effort, did not immediately return a call seeking comment Monday, nor did William Robitzek, an attorney and vocal member of the group Friends of Great Diamond Island, which strongly opposed the project.

Robitzek’s group argued the influx of transient vacation visitors to the almost entirely privately controlled island would upend its neighborhood atmosphere. The island has very few year-round residents, but is more populated in the summer.

On Monday, Davis said the fire marshal’s office still had yet to determine if the remains of the barracks building are stable enough for investigators to launch a full-scale search for the cause of the blaze — and whether there’s a great enough likelihood that a cause could be determined to justify the cost of transporting heavy equipment to the site to help in that search.

“We’re out here now checking the building to see if we think it’s going to be safe to get into,” he said. “We haven’t made a determination yet.”

Davis said “overhead hazards falling on us, the weight of the walls coming down, steel I-beams and things like that” cause great concern for investigators, who already face uphill battles figuring out what causes fires when almost all the evidence is routinely destroyed.

“We have to weigh all those factors before we can proceed any further,” he said.

Later Monday, Portland Fire Chief Jerome LaMoria said the investigation could take “days or even weeks” to complete.

“At this point the city has determined the structure is unsafe; the next step is for the developer to secure the property in an order to make the area as safe as possible while the investigation continues,” LaMoria said in a news release issued Monday evening.

The property, located at the street address 18 McKinley Court, is on the National Register of Historic Places.

The city of Portland acquired the historic barracks property in 2005 due to unpaid taxes and agreed to turn it over to hotel developers four years later for $1. Jeff Levine, director of Planning and Urban Development for the city, on Saturday called the redevelopment effort “a wonderful reuse of a long-abandoned property. While the future status of this project is now unknown, we remain optimistic with regard to development of the site.”

Portland City Manager Mark Rees on Monday reiterated the city’s commitment to the project.

“The army barracks is an important part of Portland’s history. The city wants to work with the owner for the best possible outcome in response to this tragic fire,” he said in the release.