MEMPHIS, Tenn. - A deadly explosion at a military flare plant in West Tennessee was an accident and not caused by a criminal act, federal investigators said Thursday.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said Saturday's explosion at the Kilgore Flares plant in Toone occurred in a mixing room. The preliminary findings showed the blast was accidental, but they did not include an exact cause. Records show that six people have died in explosions or fires at the plant since 1993.

The latest explosion severely burned Michael Chism on more than 90 percent of his body. Chism was hospitalized and died on Monday. Residents of Toone, a town of about 350 people located about 60 miles east-northeast of Memphis, planned a vigil for Chism on Thursday night.

Kilgore makes air and naval decoy flares for the military, including infrared ones that help aircraft and ships counter the threat of guided missiles. Components used in production are highly flammable.

Kilgore has a 264-acre plant and a 242-acre test site in Toone, according to its website. Kilgore was founded in 1920 and became a major military contractor during World War II. It is owned by Chemring Group PLC, a British company that bought it in 2001.

In September 2010, three people were severely burned in a flash fire at the plant. One victim died a year later. Investigators found no definitive cause for that explosion, but their report says the problems were likely made worse by employees overloading their work stations with flammable materials. The company was cited for 14 violations and fined $348,000.