KANSAS CITY, Mo. (AP) — Federal investigators say they have determined that the fatal explosion at an ammunition plant in Missouri is non-criminal in nature while their probe continues to pinpoint the exact cause of the blast.

The National Response Team of the Bureau of Alcohol, Tobacco, Firearms and Explosives says in a news release Friday that they have now concluded the on-scene phase of the investigation at the Lake City Army Ammunition Plant in Independence.

Investigators have spent the past three days examining the scene and interviewing first responders and plant personnel.

Tuesday’s blast killed Lawrence Bass, of Blue Springs, Missouri, and injured four other workers.

The explosion occurred in a building where chemicals are mixed.

Contractor Orbital ATK operates the plant where it makes and tests small-caliber ammunition for the military.