ROCKTON, Ill. (WAND) - A massive fire at a Rockton chemical plant has been ruled accidental by responders. 

The Rockton Fire Protection District said the fire at the Chemtool plant occurred on the morning on June 14 as a project was happening at the plant to replace insulation on an elevated heat transfer piping network, which was part of the site's system for heating certain vessels used in manufacturing lubricating greases. The piping network carried heated mineral oil. 

Responders said an employee of an outside contractor was working on the insulation replacement project in the area of origin while using a scissor lift in order to access to piping network. 

Shortly before 7 a.m., crews said a release occurred from the elevated piping while the outside contractor employee was working. An unknown amount of mineral oil then fell and pooled on the floor in the area. 

Chemtool operators immediately saw the release and shut down the boiler. The fire ignited as employees were working on placing containment booms and de-pressuring the heat transfer piping network. 

Firefighters believe the "most credible scenario" at this time is the scissor lift hit a valve or another piece of piping with enough mechanical force to cause the mineral oil to come out. 

The source of ignition has not yet been determined. 

After the fire occurred, responders evacuated a one-mile radius around the plant as a precaution. Responders directly evacuated about 150 homes. Residents were allowed to return to their homes on June 18. 

At one point, responders allowed the fire to burn due to concerns about an environmental catastrophe with chemicals entering the nearby Rock Rover.Â