Crews are performing a controlled release of toxic chemicals from a derailed train in north-eastern Ohio, hours after ordering residents to evacuate the area or risk death.
Officials are releasing vinyl chloride into the air from five derailed rail cars that are at risk of exploding.
An evacuation order is in place for anyone within a one-mile (1.6km) radius of the site in East Palestine.
Residents were advised to stay away as of 22:00 (03:00 GMT) on Monday.
Images from the scene showed huge plumes of black smoke rising into the air shortly after officials said the controlled release would begin.
The train, which derailed on Friday, was carrying hazardous material.
On Sunday, Governor Mike DeWine said "a drastic temperature change has taken place in a rail car, and there is now the potential of a catastrophic tanker failure which could cause an explosion with the potential of deadly shrapnel traveling up to a mile".
He asked those who had not yet left to "immediately evacuate".
Sheriffs went door-to-door to count the remaining residents and urged people within the evacuation area to leave.
"We will be enforcing the evacuation zone," Sheriff Brian McLaughlin of Columbiana County said in a statement. "Please, for your own safety, remove your families from danger."
Schools and village offices were closed on Monday and businesses within the evacuation zone were not allowed to open.
A shelter-in-place order was put into effect for the whole town of East Palestine, which is home to roughly 5,000 people, over concerns about chemicals possibly spilling into the air.
Two evacuation stations have opened to provide shelter to residents.
Officials from the National Transportation Safety Board (NTSB) said there were 20 rail cars carrying hazardous material on the train. Ten of them derailed about 15 miles south of Youngstown, Ohio.
Five of the derailed cars were carrying the chemical vinyl chloride, a type of gas.
Exposure to vinyl chloride has been associated with heightened risks of certain forms of cancer, according to the National Cancer Institute - but state environmental officials said harmful levels had not been detected in the community.
It is not yet known why the Norfolk Southern train, which was carrying 100-plus cars, derailed. It departed from Madison, Illinois, and was bound for Conway, Pennsylvania when it came off the tracks, according to the NTSB.
Officials said it would take four to six weeks for the NTSB to produce a preliminary report on the accident.
