The fire that caused a massive explosion at a Texas fertilizer plant in 2013 was a criminal act, federal and state authorities announced Wednesday.

Fifteen people — 12 of whom were firefighters and first responders — were killed in the blast at the West Fertilizer Plant in West, Texas.

The Bureau of Alcohol, Tobacco, Firearms and Explosives and the Texas State Fire Marshal's Office made the announcement at a news conference in West. ATF investigators said the agency's fire research lab determined that the fire was intentionally set.

"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," the ATF said in a statement. "All viable accidental and natural fire scenarios were hypothesized, tested, and eliminated."

The ATF said the investigation, which so far has cost $2 million, is ongoing and offered a $50,000 reward for information leading to the arrest of the person or persons responsible for the fire.

"It is our highest priority to give victims of this tragedy an accurate explanation of what happened that day," said ATF Houston Special Agent in Charge Robert Elder.

"Why the fire was set I don't know. I can't say what their intent was," he said at the news conference, according to the Austin American-Statesman, which adds:

"He said Bryce Reed, a former West paramedic who was sentenced to 21 months in prison in 2013 on charges related to the construction of a homemade pipe bomb, is not considered a suspect, Elder said."

As the Two-Way has previously reported, a fire was reported at the plant on the evening of April 17, 2013. About 20 minutes later, the building exploded, leaving a crater 93 feet wide and 10 feet deep. Hundreds of people were injured and residents who lived nearby were forced to evacuate their homes, the Two-Way reported. According to The Associated Press, the explosion was so strong, it "registered as an earthquake of magnitude 2.1."

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The presence of powerful chemicals — including ammonium nitrate, the dangerous explosive used in the 1995 Oklahoma City bombing — contributed to the blast, the Two-Way reported.

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