(CNN) -- Several people were injured in a "flash fire" at a plant in west-central Tennessee that makes decoy flares for the military, officials said Tuesday.
The incident occurred at Kilgore Flares Co. in Hardeman County, about 75 miles northeast of Memphis, said Jeremy Heidt, spokesman for the Tennessee Emergency Management Agency.
Three hospitalized employees received burns and one was treated for stress, the company said. But hospitals said they were treating six people, three of them in critical condition.
The plant in Toone, Tennessee, makes materials that are flammable but not explosive, Heidt said.
United Kingdom-based Chemring Group, which owns Kilgore Flares, said the fire started at 12:30 p.m. in one of its "expendable countermeasures assembly facilities."
"Due to the nature of the materials involved, and its proximity to other materials, the emergency services consider it is best to let the fire burn itself out, Chemring said in a statement. "Once the fire is extinguished, an investigation into the cause of the incident will be undertaken in cooperation with the local authorities."
The Regional Medical Center at Memphis has received three patients, two of them flown in, said executive secretary Jackie Harris. All three were in critical condition with unspecified injuries, she said.
Three others were at Bolivar General Hospital in good condition, said spokeswoman Kay Cranford of West Tennessee Healthcare.
A receptionist who answered the phone at Kilgore said no local spokesman was available.
According to its website, Kilgore Flares makes decoy flares for aircraft and naval forces, many of which are used as countermeasures against heat-seeking missiles. Kilgore Flares is one of the county's largest employers, Hardeman County Mayor Willie Spencer said.
A nearby elementary school was locked down after the early afternoon incident and several hundred Kilgore employees were sent home, Spencer said.
Kilgore Flares was levied $200,000 in penalties for several violations after an employee was fatally burned in a 2001 explosion and fire, according to Occupational Safety & Health Administration records.
CNN's Aaron Cooper and Phil Gast contributed to this report.