The state fire marshal department said that investigators interviewed "almost 300 people," and followed 160 leads in their initial investigation.
In May 2013, the Texas Department of Public Safety instructed the Texas Rangers and the McLennan Sheriff's Department to join the Texas Fire Marshall's Office and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, in the criminal investigation into the explosion.[54][55]
Ultimately, investigators blamed stocks of ammonium nitrate fertilizer stored in a bin inside a seed and fertilizer building on the property for the explosion[56] but failed to identify what started the actual fire that led to the explosion.
On April 22, 2014, the U.S. Chemical Safety and Hazard Investigation Board released the preliminary results of its investigation into the explosion. It blamed the disaster on company officials' failure to take basic steps regarding safe storage of the chemicals in its stockpile, as well as inadequate federal, state and local regulations regarding the handling of hazardous materials.[57]