Ammonium nitrate stores exploded at Texas plant: state agency AUSTIN, Texas (Reuters) - Investigators have confirmed that ammonium nitrate stored at a West, Texas, fertilizer plant detonated in the devastating explosion that left 14 people dead and about 200 injured last month, the Texas state fire marshal's office said. The actual cause of the fire and subsequent blast at the West Fertilizer Co facility is still being determined, investigators said. The fire marshal's office has been leading the investigation of the April 17 blast, along with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. Investigators had hoped to determine by May 10 what caused the explosion, Texas Fire Marshal Chris Connealy said at a state legislative hearing last week. But the fire marshal's office said this week that the investigation will take one or two weeks beyond that. "We're not going to leave anything unturned on the scene, and there just happened to be more that needed to be turned," Assistant State Fire Marshal Kelly Kistner said on Tuesday. The blast caused an estimated $100 million in damages to homes, businesses and schools near the fertilizer plant. The dead included 11 firefighters and other first responders who had rushed to contain a fire at the plant. It can be combustible under certain conditions, and was used as an ingredient in the 1995 bombing of a federal building in Oklahoma City that left 168 people dead. Investigators believe the fire started somewhere in the 12,000 square foot (1,100 square meter) fertilizer and seed building. They continue to work on pinpointing the exact location. "The number-one issue that we're trying to resolve right now is, where did the fire start and how did the fire start," Kistner said. Investigators said they have eliminated the following causes for the initial fire: weather, natural causes, anhydrous ammonia, a railcar containing ammonium nitrate, and a fire within the ammonium nitrate bin. Writing by Carey Gillam and Corrie MacLaggan; Editing by Marguerita Choy and Tim Dobbyn