Officials in West, Texas believe fire was set in 2013 plant explosion @abc13houston #abc13 pic.twitter.com/k7HvnYfnsy — Christine Dobbyn (@ChristineDobbyn) May 11, 2016

"It is our highest priority to give victims of this tragedy an accurate explanation of what happened that day" in West, ATF Houston SAC — ATF HQ (@ATFHQ) May 11, 2016

WEST, TX -- An intentionally set fire caused the 2013 explosion at a Texas fertilizer plant that killed 15 people, federal officials said Wednesday, saying the fire was "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 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.Alcohol, Tobacco, Firearms and Explosives 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. The U.S. Chemical Safety Board adopted recommendations earlier this year that federal regulators set higher standards for safe handling and storage of fertilizer-grade ammonium nitrate.Federal regulators say the way the fertilizer was stored, with combustible materials nearby, and the lack of ventilation were contributing factors to the detonation. But they also cited a failure to conduct safety inspections of the plant, shortcomings in emergency response such as with hazmat training, and poor land planning that allowed development to sprout around the plant over the years.Among those killed in the explosion were 12 emergency personnel, primarily ones with the West Volunteer Fire Department who responded to the initial blaze.The Chemical Safety Board report approved by the board in January said the response to the fire was flawed for various reasons, including for not establishing an incident command center and a lack of understanding about the possibility of a detonation.The report also noted that Texas had 80 plants that stored more than 5 tons of ammonium nitrate, a chemical used in fertilizer, and that 19 plants storing fertilizer-grade ammonium nitrate operated within a half-mile of a school, hospital or nursing home.The West plant "was about 550 feet from the closest school, which sustained catastrophic damage as a result of the explosion, which could have resulted in additional loss of life had the school been in session at the time," the report noted. That explosion caused about $100 million in property damage, according to the Texas Department of Insurance, and insurance-related losses were approximately $230 million.