An intentionally set fire caused the 2013 explosion at a Texas fertilizer plant that killed 15 people, federal officials said Wednesday, calling the fire "a criminal act." Federal and state investigators said no arrests have been made, but that they are investigating who was responsible for causing the fire at the West Fertilizer Co. facility in the town of West on April 17, 2013, the AP reports. The fire caused ammonium nitrate to ignite, triggering a massive explosion that also injured hundreds of people and left part of the small town in ruins, and marked one of Texas' worst industrial accidents. ATF special agent Robert Elder said investigators came to the conclusion after ruling out other reasonable causes, but he didn't release specifics or a possible motive. Inspectors have previously said three possible scenarios caused the fire: faulty electrical wiring, a short circuit in a golf cart stored at the plant, or arson.
"We have eliminated all reasonable accidental and natural causes," Elder said during a news conference. "This was a criminal act." The ATF also released a statement saying the fire had been ruled "incendiary," or intentionally set, after investigators reviewed more than 400 interviews, a fire-scene examination, witness photos and videos, and "extensive scientific testing" at an ATF fire research laboratory. Federal regulators had previously issued a report that found inadequate emergency response coordination and training and careless storage of potentially explosive materials contributed to the blast, which left a crater 90 feet wide and 10 feet deep. Among those killed in the explosion were 12 emergency personnel, primarily ones with the West Volunteer Fire Department who responded to the initial blaze. Authorities are offering a $50,000 reward for information leading to an arrest in the 2013 blaze, the Houston Chronicle reports.