A number of potential causes have been eliminated, including kindling from prior fires, spontaneous ignition, the storage of ammonium nitrate, smoking at the site, weather, and the 480 volt electrical system.
Potential causes that could not be eliminated included the smaller 120 volt electrical system, golf cart at the site that had been recalled by its manufacturer, and an intentionally set fire.
The Fire Marshal's office refused to speculate about the arrest of first responder Bryce Reed for an explosive device, and would not take any questions on him.
15 people were killed in the explosion at a fertilizer plant on April 17, most of them first responders.
The investigation is ongoing.
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