After completing their on-scene examination to determine the cause of Tuesday's fatal explosion at the Lake City Army Ammunition Plant, investigators have determined the blast was accidental, according to the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

In a release Friday, the ATF said the overall investigation is still ongoing. The explosion killed one worker and injured four. The National Response Team of the ATF and investigators from the U.S. Army Criminal Investigations Command have examined the scene, and conducted interviews with first responders and plant personnel thus far.

The explosion, which killed longtime worker Lawrence Bass, 55, of Blue Springs, occurred in a primer mixing building. Bass was an employee of Orbital ATK, the contractor that operates the 4,000-acre complex. Four other workers injured by the explosion were evaluated at the scene and refused transport for further treatment.

Lake City, which was established in the U.S. buildup for World War II, manufactures small-caliber ammunition for all U.S. military branches.

Also Friday, Orbital ATK said all full-time bargaining unit employees who had already returned to work at Lake City, or are otherwise scheduled to return, will return Wednesday. Shift schedules will vary by operation next Wednesday through Friday, so employees should call their building hotline number for specific information and reporting instructions.

Orbital ATK said it will have grief counselors available onsite beginning Wednesday.