The facility caught fire on Wednesday, April 17, 2013, then exploded with the force of 7.5-10 tons of TNT[19] as firefighters were attempting to douse the flames.[20] The cause of the initial fire remains unknown;[21] authorities ruled out weather, natural causes, anhydrous ammonia, and ammonium nitrate in a rail car as possible causes of the fire.[22] The explosion occurred about 7:50:38 p.m. CDT (00:50 UTC, April 18).[6][23]