A “criminal act” caused the 2013 fertilizer plant explosion in West, Texas, according to a report released by federal officials.   

Officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) ruled the explosion as an arson and said someone had intentionally set the fire.

“This fire was a criminal act,” Robert Elder, special agent who manages the bureau, said at a press conference Wednesday. “We have ruled out all [other] reasonable natural causes.”

Investigators spent $2 million on the investigation and interviewed 400 people, according to ATF officials. The bureau is offering a $50,000 reward for information leading to the arrest and capture of the person responsible for the arson. The agency declined to say if they knew of any suspects connected to the explosion.

The explosion, which occurred at West Fertilizer Co. on April 17, 2013, killed 15 people and created a crater 100 feet wide and 10 feet deep.  President Barack Obama visited the nearby town of Waco, Texas on April 25 in 2013 to speak at a memorial service honoring the deceased and injured.