KNOXVILLE, Tenn. - Fire officials say six workers have been injured in an explosion at a Knoxville steel mill.

Knoxville Fire Department Capt. D.J. Corcoran told media that one worker was in critical condition with burns over 80 percent of his body. He said the others suffered less severe injuries.

Investigators are trying to determine the cause of the blast Thursday night at Gerdau Ameristeel Mill in north Knoxville.

Corcoran said it occurred in a part of the plant where metal is melted down.

Nearby residents said the blast could be heard from blocks away.