Fire officials now say that a "chemical product" used in tire manufacturing "self-heated," triggering the explosion and fire that rocked Specialty Tires of America’s plant in Indiana, Pa., last week.

The material was apparently stored in a large refrigerated room where raw rubber was also stored. The plant’s cement block walls and the two steel access doors to the room helped keep the fire from spreading, officials said. The unnamed chemical “must be stored under ‘optimal’ conditions, otherwise it will heat up by itself,” according to police.

Because the plant was on summer maintenance shutdown, only a small crew of about 30 people was working at the time. The plant employs 400.

Three of those workers, though, were hurt when they attempted to put out the fire before local fire departments responded. Using the plant’s own fire equipment, the three fought the flames until they were disabled by smoke. They were treated and released from the Indiana Regional Medical Center.

Three firefighters were also treated for smoke inhalation.

Police and fire officials are continuing their investigation.