The cause of the explosion at CTA Acoustics was not immediately known. State inspectors were sent to the site.
The injured employees were taken to at least five hospitals. Dr. George Liu, a surgeon at Baptist Regional Medical Center in Corbin, said at least eight were burned on more than 80 percent of their bodies.
"We prepare as physicians for disasters I have never seen this," Liu said. "The best thing people can do is pray for these people."
Authorities shut down a 13-mile stretch of nearby Interstate 75 for about an hour and evacuated people within a half-mile radius of the plant because of concerns the smoke was dangerous. But CTA attorney Jim Tomaw said there were no hazardous chemicals in the plant, which makes acoustical and thermal insulation products for the industrial and automotive industries. Evacuated residents were allowed to return to their homes by Thursday evening.
"We don't feel there is any great danger to the public," said Joe Bradshaw, emergency management director for Knox County.
Tomaw said 150 of the company's 561 employees were in the plant at the time of the explosion, which happened near an oven where raw fiberglass is mixed with resin and molded into sheets. The company said all workers were accounted for.
"It was just like a tornado came through," said Helen Rutherford, 48, who has worked at the plant for 29 years. "I didn't know how bad it was until I got outside and saw the smoke and the fire. ... What I saw today, I never want to see again."
Roger Bales, 33, was working on a piece of equipment near where the explosion occurred. He escaped without injury.
"It happened so fast. I looked up and I saw a big ball of fire coming at me," Bales said.
A spokeswoman for the Bureau of Alcohol, Tobacco and Firearms in Louisville said the federal agency was providing routine assistance to local authorities in investigating the explosion.
"At this point we're not aware of any criminal activity," said Laura Volk.
Earlier this month, the state labor agency proposed to cite the company for an allegedly serious violation involving safety guards on machinery, agency spokesman Eddie Jacobs said.
The company also was cited in 1989 and 1993, mostly for insufficient safety guards on machinery, according to agency records.
Associated Press writer Charles Wolfe in Frankfort, Ky., contributed to this report.