WASHINGTON, May 11, 2016 -The fire that led to a massive explosion at the West Fertilizer plant in West, Texas, was set intentionally, qualifying it as a criminal act, the Bureau of Alcohol, Tobacco, Firearms and Explosives has concluded.

“After more than 400 interviews, a systematic fire-scene examination, the review of witness photos, videos and observations, as well as extensive scientific testing at the ATF Fire Research Lab in Beltsville, Maryland, the fire has been ruled ‘incendiary,’ or intentionally set,” ATF said.

“All viable accidental and natural fire scenarios were hypothesized, tested, and eliminated,” the bureau said.

The April 17, 2013, explosion, which came after a fire burned for about 20 minutes, killed 15 people, including 12 first responders. An additional 300 people were injured and more than 500 homes were destroyed. The blast left a crater 93 feet wide and 12 feet deep.

Investigators are pursuing “many leads in a complex investigation with many moving parts,” ATF Special Agent in Charge Robert Elder said at a news conference, according to Green Markets, which covers the fertilizer industry.

“ATF, along with the (Texas State Fire Marshal’s Office), Texas Rangers, McLennan County District Attorney’s Office, and numerous other agencies are asking for the public’s assistance in identifying the person or persons responsible for this tragedy,” ATF said. “Those with information should contact Waco Crime Stoppers at 254-753-4357 or online at www.wacocrimestoppers.org.”

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ATF’s Houston Field Division also announced a reward of up to $50,000 for information leading to the arrest of the person or persons responsible for the fire and subsequent explosion, which detonated about 30 tons of ammonium nitrate stored at the plant.

Up until today, investigators said there were three possible scenarios: faulty electrical wiring, a short circuit in an electrical golf cart, or an intentional act of arson.

In January, the federal government’s Chemical Safety Board completed its investigation, concluding that “OSHA efforts to oversee facilities that store and handle (fertilizer grade ammonium nitrate) fell short at the time of the incident.”