This photo from video by KCTV5 shows damage to the side of a building in the aftermath of a fatal explosion at the Lake City Army Ammunition Plant in Independence, Mo., Tuesday, April 11, 2017. (KCTV5 via AP)

This photo from video by KCTV5 shows damage to the side of a building in the aftermath of a fatal explosion at the Lake City Army Ammunition Plant in Independence, Mo., Tuesday, April 11, 2017. (KCTV5 via AP)

KANSAS CITY, Mo. (AP) — An explosion Tuesday at a sprawling ammunition plant near Kansas City, Missouri, killed one worker and injured four others, the U.S. Army said.

The blast at the Lake City Army Ammunition Plant in Independence, just east of Kansas City, occurred in a building where chemicals are mixed, Army officials. The building has been secured and rendered safe, they said, allowing investigators to begin looking into what caused the explosion.

Other explosions have occurred at the plant, including a 1990 blast that killed one worker and a 1981 explosion that severely burned a worker who later died, according to records. In 2011, six people were injured in a blast there.

The plant has been fined for workplace safety issues at least three times.

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All the plant’s nearly 1,800 employees were sent home after Tuesday’s explosion and told to call in before returning to work Wednesday. The four injured workers were evaluated at the scene and declined additional treatment, officials said.

Lt. Col. Eric B. Dennis, the plant’s commander, offered his condolences to family members of the worker who died.

“Making ammunition is dangerous work and our employees risk their lives to protect the men and women in uniform,” Dennis said. “This is the sacrifice they make to support our country and I am humbled by the ultimate sacrifice this employee made today.”

The Bureau of Alcohol, Tobacco, Firearms and Explosives will lead the investigation. Workplace safety experts with the U.S. Department of Labor’s Occupational Safety and Health Administration also will be looking into the blast.

The 77-year-old plant, created to help arm the U.S. military effort in the run-up to World War II, makes small-caliber ammunition and tests its reliability. The factory also operates the NATO test center.

The plant, which sits on nearly 4,000 acres and is the first of a dozen Army small-arms factories, has undergone hundreds of millions of dollars in upgrades since the mid-2000s. The property has more than 400 buildings and nine warehouses, and has a storage capacity of more than 700,000 square feet.

The factory has a governmental staff payroll of $2.9 million and a workforce that includes 29 Department of Army civilians and a soldier to provide contract oversight.

Dulles, Virginia-based contractor Orbital ATK, the biggest maker of small-caliber ammunition for the U.S. Department of Defense, runs the plant. Since 2000, Orbital has produced more than 17 billion rounds of small-caliber ammunition at Lake City for military purposes.

Jim Nickels, vice president and general manager of Orbital ATK, said the explosion happened in a building where workers mix chemicals into the primer that goes into all small-caliber munitions.

Orbital announced Monday that it has received a $92 million order from the Army for 5.56 mm and 7.62 mm ammunition, adding that Orbital and the Army “have made significant upgrades at the facility in recent years that have enhanced product quality; and performance, efficiency and operational improvements for safety and environmental stewardship.”

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Orbital has roughly 12,500 employees in 18 states and in several international locations.

When the 2011 explosion occurred at the plant, injuring six people, Alliant Techsystems, or ATK, was the contractor operating the facility. OSHA fined the plant for workplace safety issues that year and also in 2008 and 2012.

The largest penalty was in 2011 when Alliant was initially fined $28,000. It paid $5,600. OSHA had cited it for “serious” issues with process safety management of hazardous chemicals.