On Monday, the state fire marshal's office said it ruled out four potential causes: weather, natural causes, anhydrous ammonium, and ammonium nitrate in a rail car.

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The state fire marshal's office had previously ruled out four potential causes: weather, natural causes, anhydrous ammonium and ammonium nitrate in a rail car.

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On Monday, the state fire marshal's office said it had ruled out weather, other natural causes, anhydrous ammonium and ammonium nitrate in a rail car as possible causes.

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Forensic reports suggested substances used in the explosion had included RDX, ammonium nitrate and petroleum hydrocarbon, he added.

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They also spoke about fertiliser and compared its ammonium nitrate content with sports injury cold packs.

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Ammonium nitrate, a solid fertilizer composed of ammonia and nitrogen, is also a component of explosives widely used in mining.

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One of the worst disasters in U.S. history involving a form of ammonia occurred in April 1947 when a ship loaded with ammonium nitrate -- a solid fertlizer composed of ammonia and nitrogen -- caught fire while docked in Texas City, Texas, in April 1947.

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On 16 April, 1947, a fire on a ship loaded with ammonium nitrate set off a series of explosions in the port of Texas City, and fires that burned for days.

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