Humphreys County Sheriff Chris Davis said the blast at Accurate Energetic Systems, which supplies the military, was one of the worst scenes he's ever seen.
He did not specify how many people were killed but referred to the 19 missing as "souls" and said officials were still speaking to family.
"There's nothing to describe. It's gone," Davis said. "It's the most devastating scene that I've seen in my career."
People reported hearing and feeling the explosion from kilometres away. The company's website says it makes and tests explosives at an eight-building facility that sprawls across wooded hills in the Bucksnort area, about 97km southwest of Nashville.
Davis said investigators are trying to determine what happened and couldn't say what caused the explosion.
Aerial footage of the aftermath by WTVF-TV showed the explosion had apparently obliterated one of the facility's hilltop buildings, leaving only smouldering wreckage and the burnt-out shells of vehicles.
There's no further danger of explosions, and the scene was under control on Friday afternoon, according to Grey Collier, a spokesperson for the Humphreys County Emergency Management Agency.
Emergency crews were initially unable to enter the plant because of continuing detonations, Hickman County advanced EMT David Stewart said by phone. He didn't have any details on casualties.
Accurate Energetic Systems, based in nearby McEwen, did not immediately respond to a phone message seeking comment on Friday morning.
"This is a tragedy for our community," McEwen Mayor Brad Rachford said in an email. He referred further comment to a county official.
Residents in Lobelville, a 20-minute drive from the scene, said they felt their homes shake and some people captured the loud boom of the explosion on their home cameras.
The blast rattled Gentry Stover from his sleep.
"I thought the house had collapsed with me inside of it," he said by phone. "I live very close to Accurate and I realised about 30 seconds after I woke up that it had to have been that."
State Representative Jody Barrett, a Republican from the neighbouring town of Dickson, was worried about the possible economic impact because the plant is a key employer in the area.
"We live probably 15 miles (24km) as the crow flies and we absolutely heard it at the house," Barrett said. "It sounded like something going through the roof of our house."
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