But no arrests have been made and authorities are seeking who was responsible for the blaze.
A massive explosion which caused a tremor as powerful as a small earthquake left part of the small town of West in ruins.
Initially it was blamed on an industrial accident.
On the night of 17 April 2013, a fire broke out at the West Fertilizer Co and was followed by an explosion that possessed the force of a 2.1-magnitude earthquake.
The explosion flattened homes, shattered a block of flats and badly damaged a nursing home and several schools.
On Wednesday, the announcement that there was criminal intent was made by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the Texas State Fire Marshal's Office.
In an exhaustive investigation, which included 400 interviews and a model recreation of the plant, they eventually ruled out all other possible causes.
They had previously said faulty electrical wiring, a short circuit in a golf cart stored at the plant or arson were among the possible reasons for the blaze.
A reward of up to $50,000 has been offered for information about the culprit.
Of the 15 people killed, 12 were emergency personnel attending the fire. Another 200 people in the small town of 2,800 were injured.
The fire at a fertiliser plant in the US state of Texas that killed 15 people and wounded hundreds in 2013 was a criminal act, say investigators.