The explosion April 17 in the central Texas town of West occurred after fire broke out in the building, The Dallas Morning News reported.
"All indications" are leading to the conclusion that ammonium nitrate was the cause of the explosion, said Rachel Moreno, a spokeswoman for the State Fire Marshal's Office.
The fire started in the building where ammonium nitrate, a component of fertilizer, was stored, but not in the bin that exploded, Moreno said.
She declined to comment on whether anyone was in the building at the time of the explosion.
Investigators haven't determined if the disaster was criminal act but they haven't ruled out the possibility, Moreno said.
The April 17 blast injured 200 people and damaged or destroyed hundreds of homes.
Damage was estimated at $100 million. The company was insured for $1 million.
The explosion left a crater 93 feet wide and 10 feet.
Company officials told the state in February the facility contained up to 270 tons of ammonium nitrate.