Sarnia Fire and Rescue does not believe a large fire at the former MIG building on Christina Street was intentionally set.
Four stations were called to the blaze near the city's downtown at around 7 a.m. Friday.
Fire Prevention Officer Roel Bus said its origin was almost certainly in the attic.
"So we're still narrowing down the actual ignition source," said Bus. "It's not considered to be suspicious. We don't think it was intentionally set."
Bus said the damage was extensive in the attic, and that there was water damage throughout the rest of the structure.
He said it's hard to pinpoint a damage estimate for the historical building.
"Judging by today's prices and everything, it's probably approaching $1-million, might even go over, but I'm really hoping that the integrity of the building is okay," he said. "There was water damage but there was no fire damage essentially below the ceiling of the second storey."
Bus added that all of the fire damage was in the attic.
"And it did have a steel roof which kept the heat in the attic which kept it going kind of, and that's why it was kind of a stubborn one to get out too," he said. "But they did a great job to extinguish it."
Bus said a substantial amount of water was used but the integrity of the building itself should not be compromised.
He said the building was under construction, nobody was living at the site and that contractors had vacated the building at around 5 p.m. the day before.
A section of Christina Street was closed for several hours Friday, and fire crews remained on scene until the site was handed back over the owners Saturday morning.
