LACKAWANNA, N.Y. — Officials say that they haven’t determined a cause for a massive fire last year that engulfed a former steel mill in upstate New York.

The ruling on the Lackawanna fire followed months of investigation by the Buffalo Fire Department. Officials ruled the cause of the fire at the former Bethlehem Steel mill as undetermined on Thursday.

James Otwell, a lead investigator for the department, says he’s highly certain the fire was caused by a broken light bulb as originally reported by authorities. He tells WIVB-TV no one saw it happen, so it cannot be definitively ruled as the cause of the fire.

The blaze broke out Nov. 9 in a sprawling building that housed several businesses. Thick, acrid smoke from the fire forced hundreds of people living near the complex to evacuate their homes.