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LOUISVILLE, Ky. (AP) — A chemical fire was still burning early Thursday after an explosion at the site of a train derailment in central Kentucky that forced the evacuation of hundreds of people, including an entire small town.

Metro Louisville Emergency Management spokeswoman Jodie Duncan said hundreds remained evacuated, including those from the town of West Point and people from nearby Louisville.

The evacuation order came after a cutting torch ignited vapors Wednesday while workers tried to separate two rail cars from an earlier derailment. The vapors were from a colorless, flammable gas called butadiene.

Three workers were taken to the University of Louisville hospital with severe burns. Authorities have not released the names of the injured workers but said one was in very critical condition, another in critical condition and the third in serious condition.

"The workers that are here are highly trained and this is one of those freak accidents that occurs unfortunately," Lt. Col. Rick Harrison, assistant chief with the suburban Buechel Fire Department said.

Authorities were letting the fire burn itself out, but couldn't offer a timetable on how long that would take. On Wednesday, they had predicted it would be out in a couple of hours.

"Fire can be unpredictable," Duncan said Thursday morning. "We just don't know how long it's going to take to burn out."

She said the evacuation order would stay in place until the blaze was extinguished.

Firefighters continued spraying water onto the butadiene car and adjacent cars carrying hydrogen fluoride Thursday in order to cool them. Officials had been concerned that the fire could spread, but Duncan said the hydrogen fluoride cars were now cool to the touch.

The Paducah & Louisville Railway train derailed Monday morning near Dixie Highway. Nine of the 13 derailed cars were carrying hazardous chemicals.

Residents within a 1.2 mile radius of the wreck were evacuated Wednesday after the gas caught fire, sending up flames and thick, black smoke. Those living within a 5-mile radius were ordered to stay indoors. Also, three local schools within the areas of the evacuation or shelter-in-place orders were closed Thursday.