Investigators have determined that ammonium nitrate was the cause of the explosion at a West, Texas, fertilizer plant last month that left 14 people dead and some 200 injured, a spokeswoman for the Texas state fire marshal's office said on Tuesday.
"The investigators have been able to narrow down the origin to the fertilizer and seed building on site, and we also know that what caused the explosion was the ammonium nitrate," said Rachel Moreno, a spokeswoman for the Texas State Fire Marshal's Office. "What we don't know is exactly why."
The fire marshal's office has been leading the investigation of the April 17 blast, along with the federal Alcohol, Tobacco, Firearms and Explosives agency (ATF).
Ammonium nitrate is a dry fertilizer mixed with other fertilizers such as phosphate and applied to crops to promote growth. It can be combustible under certain conditions, and was used as an ingredient in the Oklahoma City bombing in 1995 that left 168 people dead.