In a startling development, federal investigators have determined that the deadly and destructive 2013 fertilizer plant blast in West, Texas was no accident.

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has ruled that the fire at the West Fertilizer Co. was “incendiary” or intentionally set. The ATF’s lengthy investigation into the incident, which killed 15 people and destroyed dozens of buildings, included more than 400 interviews, a thorough fire-scene examination, the review of witness photos, videos and observation and extensive scientific testing at the ATF Fire Research Lab in Beltsville, Maryland.

The ATF is offering 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.

The explosion occurred after a fire heated thirty tons of fertilizer grade ammonium nitrate (FGAN) at the storage and distribution facility. The event devastated the rural community just north of Waco, leveling 500 homes and injuring 200 people. Ten firefighters battling the fire that preceded the blast were among the dead.

In its investigation into the tragedy, the U.S. Chemical Safety Board (CSB) found that limited regulatory oversight, poor hazard awareness, inadequate emergency planning, and the proximity of the facility to nearby homes and other buildings all contributed to the incident’s severity.