Officials launched a hunt Wednesday for possible criminal suspects in the 2013 explosion that killed 15 people at a fertilizer plant in West, Texas, after announcing the fire that triggered the blast was arson.
The Bureau of Alcohol, Tobacco, Firearms and Explosives announced a $50,000 reward for information. Officials said they had made no arrests.
The fire at the West Fertilizer Co. facility in April 2013 caused ammonium nitrate to ignite, triggering a massive explosion that killed 15, injured hundreds more and left part of the small town in ruins.
ATF officials in Houston said they conducted more than 400 interviews and that the investigation had cost some $2 million dollars.
"We have eliminated all reasonable accidental and natural causes," ATF Special agent Robert Elder said. "This was a criminal act."
Inspectors previously said three possible scenarios caused the fire: faulty electrical wiring, a short circuit in a golf cart stored at the plant, or arson.
In addition, federal regulators earlier issued a report that found inadequate emergency response coordination and training and careless storage of potentially explosive materials contributed to the blast. The U.S. Chemical Safety Board adopted recommendations 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.
The April 2013 explosion caused about $100 million in property damage, according to the Texas Department of Insurance, and insurance-related losses were approximately $230 million.
Fox News' Matt Dean and The Associated Press contributed to this report.