A cargo train derailed in the midwestern United States, sparking a massive fire and triggering the release of small amounts of the hazardous chemical vinyl chloride.
No injuries or fatalities were reported after about 50 cars of the 140-car train came off the tracks late on Friday near the Ohio-Pennsylvania state border.
The Norfolk Southern train was shipping cargo from Madison, Illinois, to Conway, Pennsylvania, when it derailed in East Palestine, Ohio.
Ten of the cars that derailed carried hazardous materials, including five with vinyl chloride, the National Transportation Safety Board, the lead federal investigating agency, said in a tweet on Saturday night.
Vinyl chloride, a colourless gas, is deemed carcinogenic by the US National Cancer Institute. It is used to make the white plastic PVC pipes commonly used in plumbing.
"We have not confirmed that vinyl chloride has been released other than from the pressure release devices" installed on some cars, the NTSB tweet said.
The devices relieve a build-up of pressure in the tanker cars to prevent explosions.
Several explosions were heard as the cars continued to burn into Saturday.
"It's an active fire scene," said NTSB board member Michael Graham.
Low temperatures hampered the effort, as fire trucks pumping water froze up.
Firefighters wore hazmat suits as they tackled the blaze.
Roughly 2,000 residents — about half of the town's population — were asked by authorities to evacuate their homes.
Officials asked anyone living within about 2 kilometres from the scene to leave. They also enforced a shelter-in-place order for the entire town.
"We cannot stress enough that we need everyone to stay away from the scene," East Palestine's town manager wrote in a letter posted on Facebook.