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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The main reason people want to get hold of ammonium nitrate is for use in agriculture and to that end, says Mr Whitty, an "effective system" is in place for suppliers to inform the authorities of any irrregular and sudden inquiries about buying large amounts.

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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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