One worker was killed and four others were injured in an explosion at the Lake City Army Ammunition Plant in Independence, MO. The plant, which is responsible for making small-caliber ammunition for the military, also runs the NATO test center and tests the reliability of the bullets there.

According to the Army Times, the cause of the explosion, which occurred in a primer mixing building, is currently unknown. Army officials said the building itself has been rendered safe, and will be under investigation by the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Medical professionals examined and released the injured on the scene. The plant sent all employees home after the explosion on Tuesday, and told them to call in before heading to work on Wednesday.

While the plant has been rendered safe as of now, it has a long history of dangerous problems. One worker died in a primer mixture explosion in 1990. Six people suffered injuries in a 2011 explosion, and the plant received fines for safety issues from the Occupational Safety and Health Administration (OSHA) in 2008, 2011, and 2012.

Lt. Col. Eric B. Dennis, who took over command of the plant in July 2016, managed a revitalization project to improve the safety and security of the facility. Unfortunately, the modernization did not extend to the building in which the explosion occurred. Dennis extended his condolences to the family of the deceased, stating he was “humbled by the ultimate sacrifice this employee made today.”

Today, we, along with Dennis, think of the brave men and women who work tirelessly to create small-arms munitions for our country.