Investigators have announced that the cause of the West Fertilizer Company explosion has been ruled undetermined. This ruling is made when the cause can’t be proven to an acceptable level of certainty – which may be due to insufficient information or the inability to eliminate multiple causes. The Bureau of Alcohol, Tobacco and Firearms did eliminate some possible causes for the fire – including rekindling from a previous fire, spontaneous ignition, a 480-watt electricity system, anhydrous ammonia, ammonium nitrate, smoking and weather concerns. A 120-volt electrical system, a golf cart and an intentionally set fire were not eliminated as possible causes. While the investigation into the fire’s cause is closed – a separate criminal investigation remains open.