Last weekend’s fire that heavily damaged a Dresden housing unit for people receiving treatment for substance abuse and mental illness probably was caused by someone who was smoking, investigators from the state fire marshal’s office said Friday.

The fire began early Sunday morning in the back of the large, barnlike structure at 633 Gardiner Road, authorities said. That part of the building was a designated smoking area and also where electricity entered the structure.

After inspectors determined this week that electrical problems did not lead to the blaze, they now believe that a smoking accident caused the fire, Steve McCausland, spokesman for the Maine Department of Public Safety, said in a news release.

More than five dozen firefighters from Dresden and 10 other communities went to fight to the blaze, which was contained to the attic and roof but did enough damage to leave 13 people homeless.

The Freedom Center, as the nonprofit operation was known, provided private bedrooms and bathrooms to those receiving treatment. The American Red Cross of Maine assisted those who lost their housing in the fire.

Jan Burns, executive director of the Freedom Center, was not at the site at the time of the fire; but her dog, Snickers, was in the building and died, authorities said.