The fire that killed 15 people at a fertiliser plant in Texas in 2013 was a criminal act, federal authorities have said.
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 Fertiliser Co facility on April 17 2013, in the town of West.
The fire caused ammonium nitrate to ignite, triggering a massive explosion that killed 15 people, injured hundreds and left part of the small town in ruins.
Alcohol, Tobacco, Firearms and Explosives special agent Robert Elder said investigators came to the conclusion after ruling out all other reasonable causes.
Inspectors had previously said faulty electrical wiring, a short circuit in a golf cart stored at the plant, or arson could have caused the fire.
Mr Elder said: "We have eliminated all reasonable accidental and natural causes. This was a criminal act."
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.
The US Chemical Safety Board adopted recommendations that federal regulators set higher standards for safe handling and storage of fertiliser-grade ammonium nitrate.
Federal regulators said the way the fertiliser was stored, with combustible materials nearby, and the lack of ventilation were contributing factors to the detonation.
They also mentioned 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 said the response to the fire was flawed for various reasons, including for not establishing an incident command centre 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 tonnes of ammonium nitrate, a chemical used in fertiliser, and that 19 plants storing fertiliser-grade ammonium nitrate operated within half a 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 $100m in property damage, according to the Texas Department of Insurance, and insurance-related losses were approximately $230m.