Rescue workers were called Wednesday afternoon to a blast at the Rio Ammunition plant in McEwen, about 55 miles west of Nashville, Bureau of Alcohol, Tobacco, Firearms and Explosives spokesman Michael Knight said.
Knight said agents were still trying to determine exactly what happened. He said more information would be released later Thursday.
A call to Rio Ammunition on Thursday morning rang unanswered. According to its website, Rio assembles a wide range of hunting and target loads.
"The explosion did considerable damage to the facility," Humphreys County Emergency Management Agency Director Odell Poyner said Wednesday.
Poyner said authorities received reports that about 20 employees were in the building at the time of the explosion, which blew out two of the walls and a large portion of the roof.
Three men and one woman were taken to Vanderbilt University Medical Center, where one was pronounced dead, hospital spokesman Craig Boerner said. He said the survivors had blast injuries that ranged in severity from minor to critical.
Firefighters from both Humphreys and Hickman counties battled the blaze at the plant, located in a rural area right near the line dividing the counties, Poyner said. The fire ignited several brush fires in a nearby forested area. All had been extinguished by Wednesday night, he said.