The fire that led to the deadly explosion at the West Fertilizer Co. plant in April 2013 has been determined to be arson, according to the U.S. Bureau of Alcohol, Tobacco, Firearms & Explosives.

The explosion that was started by the fire killed 15 people and destroyed or damaged buildings within a 37-block radius, including the local high school.

The agency arrived at its conclusion after conducting more than 400 interviews, did a fire-scene examination and reviewed videos and witness photos.

“All viable accidental and natural fire scenarios were hypothesized, tested and eliminated,” the agency said in a statement.

As a result of the findings, the agency is now offering a reward of $50,000 to help find the person who started the fire, which is now being considered a crime. So far, no arrests have been made and it is unknown what the motive was for setting the fire.

In its earlier investigation, the U.S. Chemical Safety Board said in April 2014 that the explosion was “preventable” because the ammonium nitrate was stored in wooden buildings and bins instead of concrete buildings and the facility did not have modern fire-suppression equipment.

Litigation is ongoing in the case. CF Industries Holdings Inc., the largest producer of nitrogen fertilizer in the United States was sued by the city of West. Some of the lawsuits against West Fertilizer Company have already been settled out of court for undisclosed amounts. The next trial is slated to begin May 16.

The Agricultural Retailers Association said it and its member companies remain committed to community and employee safety through the secure and responsible handling of ammonium nitrate and other essential crop production nutrients. 

In an effort to further enhance a culture of safety and accountability within the fertilizer industry, ARA and The Fertilizer Institute established ResponsibleAg. Using third-party audits and other tools, ResponsibleAg assists agricultural retailers with federal regulatory compliance surrounding the safe handling and storage of fertilizer.

"Safety is our top priority. Through ResponsibleAg and other efforts, the agricultural retail industry continues to advance initiatives to improve upon its sound stewardship practices," ARA said in a statement.