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14 people were killed and some 200 injured in the explosion.
Ammonium nitrate, which was used as an ingredient in the Oklahoma City bombing in 1995, was the trigger for the deadly explosion at a West, Texas, fertilizer plant last month.
"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, Reuters reported.
"What we don't know is exactly why."
The blast on April 17 killed 14 people, many of them first responders, and injured 200. It also caused an estimated $200 million in damages to homes and businesses, the Houston Chronicle reported.
More from GlobalPost: Before Texas plant exploded: What did regulators know?
The area where the ammonium nitrate was stored is now a 90-foot-wide crater, Moreno said, AP reported.
She said the investigation continues and is expected to take up to two more week to complete as investigators comb through plant wreckage and the devastated neighborhood.