Investigators still don’t know the cause of the fertilizer plant explosion in West, but they have ruled out two widely discussed culprits: tanks of anhydrous ammonia and a rail car carrying dry

Officials with the State Fire Marshal’s Office confirmed Wednesday that the four tanks of ammonia at the West Fertilizer Co. plant never ruptured in the April 17 blast.

They also said a rail car found at the site laden with ammonium nitrate did not explode but was blown off the company’s rail spur when the blast occurred. County officials and the chairman of the Texas Commission on Environmental Quality had speculated the rail car 

The state fire marshal’s office was working Wednesday with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives to piece together the cause of the fire and the materials on hand that might have caused the blast.

Investigators said they had not proved whether dry ammonium nitrate stored in a building at the plant was the cause.

Ammonium nitrate is known to be explosive in extreme heat or when mixed with petrochemicals.

As of the end of December, West Fertilizer filed a government report 
showing it had 270 tons of ammonium nitrate stored at a building west of the office.

The report also showed 55 tons of anhydrous ammonia in 12,000-gallon reinforced steel tanks just south of the dry fertilizer mixing area.

Anhydrous ammonia, a liquid nitrogen fertilizer that turns to gas when released from pressure, is known to ignite under certain conditions but is not commonly associated with explosions, state chemist Tim Herrman has said.

Waco Deputy Fire Chief Gary Davis said the tanks were releasing gas through safety valves when Waco firefighters arrived shortly after the blast.

He said firefighters prepared to set up a stream of water on the tanks to keep them from overheating and rupturing, but they discovered there was no water left in the hydrants to spray.

Davis said at that point, emergency workers began to evacuate a larger area, fearing another explosion that never occurred.

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