Damage inside Green Bay Preble High School is seen Aug. 10, 2014, two days after a fire.

Spontaneous combustion started Preble fire

Firefighters say heat built up in a garbage can where soiled rags were left.

Firefighters battle a fire at the Preble High School gym in Green Bay, Aug. 8, 2014.

Preble won’t be ready by Sept. 2

Investigators have determined the fire was a result of spontaneous combustion of cleaning materials.