Story highlights Hospital spokesman says 49 people sought treatment

Cause of the derailment is not immediately clear; it is under investigation

Car was carrying a chemical compound used to manufacture plastics

(CNN) A derailed train car burst into flames early Thursday, releasing toxic fumes and forcing the evacuation of up to 5,000 people in Tennessee, authorities said.

According to Josh West, a spokesman for Blount Memorial Hospital, 49 people sought treatment, of which 25 were admitted to the hospital. The group was a mix of first responders and citizens.

The car, which was part of a nearly 60-car train, was carrying acrylonitrile, a chemical compound used to manufacture plastics, said Kristin Seay, a spokeswoman for CSX, which operates the train.

The train was headed from Cincinnati to Waycross, Georgia, when the one car veered off the rails in Blount County, just south of Knoxville.

"We have about a mile and a half safe zone. We've evacuated all of those folks," said Greg McClain, city manager for Maryville.

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