Arson investigators have determined last week’s massive recycling-plant fire was started by a backfiring propane-fueled forklift. The machinery backfire ignited several pallets of loose paper.

Knoxville Fire Department Captain DJ Corcoran said the blaze started about 1 p.m. Eastern Time Wednesday. Employees initially tried to extinguish the flames themselves. But after they went through a few fire extinguishers, the fire was getting ahead of them.

Nearby residents called 911 when they saw thick black smoke emerging from the plant. The smoke and fumes from burning plastic, rubber and chemicals forced the evacuation of dozens of nearby homes.

The blaze took firefighters more than two days to extinguish completely. And when it was out, the fire had burned through more than two acres of the sprawling plant.

The plant owed hundreds of thousands of dollars in back taxes and OSHA fines.