More than 400 people have been interviewed by investigators, but no arrests have been made three years after the explosion.
An explosion that killed 15 people and injured hundreds of others three years ago at a Texas fertiliser plant was caused by arson, officials have said.
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said the fire was "incendiary" and "intentionally set", as they announced a $50,000 reward for information leading to an arrest.
The 17 April 2013 blaze caused ammonium nitrate to ignite, triggering an explosion that flattened part of the town of West, about 75 miles south of Dallas.
Investigators said they had ruled out the possibility the blaze was sparked by a short circuit in a golf cart stored at the West Fertilizer Co facility.
During the $2m inquiry, the fire was reenacted at a Maryland research laboratory.
ATF special agent in charge Rob Elder told a news conference investigators had conducted more than 400 interviews.
Twelve emergency personnel were among those killed in the disaster.
Kelly Pustejovsky, the widow of one of the killed firefighters, said: "I pray to God that they get caught and get the death penalty.
"No life deserves to be lost due to someone's stupidity."
Investigators concluded that if the 7.30pm blast had happened earlier in the day hundreds more people would probably have been killed or injured.
Some 500 homes, schools, apartments and a care home were destroyed in an area spanning 37 blocks.
The US Chemical Safety Board and the state fire marshal's office have previously issued reports highlighting safety lapses at the plant.
Regulators said fertiliser storage - with combustible materials nearby - coupled with a lack of ventilation were contributing factors in the blast.
The explosion caused about $100m in damage, with another $230m in insurance-related losses.
But the plant reportedly only carried a $1m liability insurance policy.