The West Fertilizer Company explosion on April 17, 2013 was a “criminal act,” investigators announced Wednesday following a $2 million investigation, according to the Austin American-Statesman.

The investigation is “headed in the right direction,” said U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) special agent Rob Elder. “We’re on the right path. We’re asking for the public’s help in getting across the finish line.”

The ATF announced a $50,000 reward for information leading to arrests in the case.

Elder said he is not prepared to name a motive.

“Why the fire was set, I don’t know,” he said at a news conference. “I can’t say what their intent was.”

He said investigators came to their conclusion over several months. Investigators spoke with more than 400 people and ruled out accidental and natural causes, he said.

The 2013 explosion caused more than $100 million in property damage. Twelve of the 15 killed were firefighters or first responders.

A fire in a wood-frame building at the fertilizer company caused about 30 tons of ammonium nitrate to explode.