Investigators have determined that ammonium nitrate was the cause of a massive explosion at a fertiliser plant in the US state of Texas last month.
Fourteen people died and about 200 were injured in the explosion in the town of West on 17 April.
Ammonium nitrate is a dry fertiliser mixed with other fertilisers such as phosphate and applied to crops to promote growth.
It can be combustible under certain conditions. It was used as an ingredient in the Oklahoma City bombing in 1995 that left 168 people dead.
"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, along with the federal Alcohol, Tobacco, Firearms and Explosives agency.
