Ammo waits to be loaded in magazines for the M4 Qualification round on Range 31A at the 2013 Army Reserve Best Warrior Competition, Fort McCoy, Wis., June 26, 2013. Photo via DoD

One person is dead and at least three others are injured after an explosion at a military ammunition plant in Independence, Missouri, a U.S. Army spokeswoman told the Associated Press.

The site of the explosion, the Lake City Army Ammunition Plant, functions as a test center for the North Atlantic Treaty Organization and is the U.S. military’s main supplier of practice rounds, according to the Kansas City Business Journal.

In a statement to the media, Army Joint Munitions Command said the explosion "occurred inside a mixing building."

That plant, established in 1940, was the first of a dozen small-arms plants run by the Army since World War II. Since 2000, more than 17 billion small-caliber rounds have been manufactured at that one facility, which sits atop nearly 4,000 acres in Independence.

Managed by aerospace manufacturer Orbital ATK, the Lake City plant has proven an essential fix for the armed forces' surging demand for small-caliber ammo during the early years of the Global War on Terror. In 2011, industry magazine Small Arms Defense Journal dubbed the plant an "ammunition mecca in middle America":

A lot has changed since 1940, and yet the plant is remarkably simple. Raw materials come in, and finished ammunition goes out. Nearly everything is manufactured on site – primers, brass cartridges, and projectiles are all made on site, in a variety of sizes and calibers. Four types of ammunition are manufactured on site – 5.56mm, 7.62mm, .50 BMG and 20mm. The small arms ammunition – 5.56mm, 7.62mm, and .50 BMG – are produced from raw materials, while the 20mm ammunition is assembled from components delivered from off site. In the ten years of operation, ATK has delivered 10 billion rounds of ammunition to the U.S. Army. As a testament to the importance of the Lake City Facility, Army Chief of Staff Gen. George W. Casey Jr. visited the plant in October 2009. “I’m out here on my first trip to Lake City Army Ammunition Plant, and I’m hugely impressed with what I’ve seen here. Lake City Army Ammunition Plant is a critical manufacturer of small caliber ammunition. We are using 20 million rounds per year in Afghanistan.” Casey said.

ATK had just closed a $92 million order for small-caliber ammunition the day before the explosion, the Business Journal notes.

This is a developing story. Check back for more updates.