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.

Glasgow Warriors head coach Townsend, 43, previously worked with Scotland as part of Andy Robinson's backroom staff.
The former Scotland fly-half, who won 82 caps - plus two for the Lions - led Warriors to the Pro12 title in 2015.
"I am incredibly proud. It is a real honour to be given the responsibility of coaching the national team," said Townsend.
"My focus is firmly on the season ahead with Glasgow and I will be doing all I can to help the club continue to progress this year."
New Zealander Cotter, 54, has been in charge of the Scots since 2014 and led them to the World Cup quarter-finals last year.
Shortly before that tournament, the former Clermont Auvergne head coach had extended his contract until 2017.
Scotland lost all five Tests in his first Six Nations but they improved this year with wins over Italy and France, scoring a best-ever 11 tries in a single campaign.
"I believe we have moved forward by working to execute the simple things well," said Cotter, who has now coached in Europe for a decade after an eight-year spell in France with Clermont Auvergne.
"The focus has been to respect tradition but also perform in the present, be adaptive and innovative. I have been really pleased with the progress that has been made."
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Scottish Rugby chief executive Mark Dodson thanked Cotter for his "considerable impact" but said the time was right to promote "one of the most sought after coaches in the game", two years out from the next World Cup in 2019.
"The time is right to take the development of the national team forward with Gregor," said Dodson.
"Gregor's playing record showed he is a man of vision and great professionalism, which he has since successfully transferred into his coaching roles.
"It is these factors which I feel will enable Scotland to continue its forward momentum, building on the strong foundations laid by Vern, with the current group of players and future Scotland internationals."
Dodson has now launched a recruitment process to find Townsend's replacement at Glasgow for next season.

Two men, aged 35 and 49, were detained for breach of the peace while a 23-year-old woman was arrested for obstructing police officers.
The incident happened at about 14:00 in Buchanan Street, while a pro-refugee rally was taking place in the same area.
A spokesman from Police Scotland: "All three people were later released."
He added: "A report will be prepared for the procurator fiscal."
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.
[NEXT_CONCEPT]
Gregor Townsend is to replace Vern Cotter as Scotland head coach in June 2017, Scottish Rugby has announced.
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Three people have been arrested following a disturbance near a demonstration in Glasgow city centre.