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WEST (CBSDFW.COM) – Federal investigators announced Wednesday they now consider a fire that led to the deadly fertilizer plant explosion to be arson.

The Bureau of Alcohol, Tobacco, Firearms, and Explosives said it came to this conclusion during the past three months.

Resident Phil Calvin said he was stunned when authorities told him the news this morning.

Calvin’s son Perry was killed in the blast when he helped fight the fire.

“The longer we sat there, the madder it made me,” said Calvin. “And it makes me mad that anybody would do something like that.”

The 2013 blast killed 15 people, injured dozens and destroyed hundreds of homes.

A $52,000 reward is now being offered for any information leading to a suspect’s arrest.

Robert Elder, the ATF’s special agent in charge in Houston, said extensive testing at their fire research lab ruled out accidental or natural causes. He said the agency spent more than $2 million and re-built part of the plant in their lab.

“I’ve been with ATF 29 years and law enforcement almost 34,” said Elder. “This is one of the largest I’ve been involved at in ATF history.”

Late Wednesday afternoon, Texas Attorney General Ken Paxton announced, “Our office stands ready and willing to assist local prosecutors in pursuing the potential criminal acts behind this catastrophe that claimed the lives of 15 Texans.”

The West Fertilizer company is now being sued after being previously blamed by the Chemical Safety Board for storing ammonium nitrate, which is the chemical that exploded, in a combustible building.

Attorneys Steve Harrison and Zona Jones, who represent many families who are suing the firm issued a statement Wednesday. “The ATF did not investigate what exploded, why it exploded, or who knew that it could explode. All of that is the subject of civil litigation to be heard by a McLennan County jury.”

Last year, Governor Greg Abbott signed a new law that requires ammonium nitrate to be kept away from flammable materials. The legislation also allows state and local fire marshals to inspect and cite firms that handle ammonium nitrate.

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