ANDERSON — Fire officials believe a trash bag left in the sun caused the Sunday blaze at Hoosier Woods Apartments that left displaced 48 residents.

Anderson fire officials said Friday they suspect a trash bag filled with greasy food debris left outside on a second-floor patio was likely ignited by the sun’s heat, sparking the fire on a day when temperatures were about 90 degrees. The fire at the two-story building, 3905 Winners Circle, Building A, is being ruled accidental.

But, Deputy Fire Chief Dave Cravens said, investigators were still trying to determine what specifically inside the trash bag caused it to combust. He said fire officials were expected to learn that by midweek, when the results of chemical analysis were known.

“We do not know what in that bag caused that fire,” said Cravens, department spokesman. “We don’t know if it was a lighter or just what.

“Even though we’ve ruled it accidental, we still want to know what in that bag caused the fire.”

He said the trash bag burned through the apartment’s wooden balcony, falling to a patio below. The apartment where the blaze originated was located on the building’s southeast corner. Cravens said the fire likely spread to a nearby wall on the second floor before falling through. From there, it burned up and under the building’s roof, spreading the blaze to other second-floor apartments. First-floor apartments were mostly damaged by water, he said.

“(The garbage bag) had to be next to something to catch it on fire and then burn up the wall and then catch the attic on fire,” said Cravens. “They’re still analyzing several scenarios.”

The damage to the building — including contents — is estimated at more than $1 million, the deputy chief said. Even though the fire wasn’t believed to be suspicious, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives is assisting AFD investigators to help pinpoint the cause.

While the circumstances surrounding the blaze’s suspected origin are very rare, Anderson fire Investigator Jeff Freeman said fires started in a similar manner aren’t unheard of. He said the right circumstances — greasy food debris, such as empty chip bags, left in a clear garbage bag — combined with the right conditions — high heat — could likely cause a fire.

“They’re not real common, no,” said Freeman, cautioning that the exact cause wouldn’t be known for certain until lab analysis was complete.

He said the apartment’s occupants had moved in a few days before the fire, and the trash bag was placed near the patio’s sliding glass door and not far away from cardboard boxes.

Freeman said other possible causes, such as electrical malfunction, had been ruled out.

The 16 residents inside the building when the fire was discovered shortly after 5 p.m. all made it out safely. Firefighters had the blaze extinguished in about an hour later. Officials with Hoosier Woods previously said the 48 displaced residents were moved into vacant apartments in the complex.

“It’s just an unfortunate deal for everybody,” Freeman said. “I feel bad for everyone involved. It’s a bad deal.”