More than 5,000 residents were evacuated after a freight train carrying a "highly flammable and toxic gas" partly derailed and caught fire overnight just outside Knoxville, Tennessee, officials said.
Ten officers were hospitalized after breathing in fumes, Blount County Sheriff's Department spokeswoman Marian O'Briant told NBC News.
One car of the train derailed and was on fire, railroad officials said.
An evacuation zone is in place over a two-mile radius from the derailment, near Maryville, and could be in place for up to 48 hours, the fire department said.
Two shelters were being opened for residents including one at a local high school, according to David Kitchen, senior disaster program Manager for the Tennessee Volunteer Region of the Red Cross.
The train was carrying acrylonitrile — which authorities described as a "highly flammable and toxic gas."
The EPA says the substance is used in the manufacture of acrylic fibers and if inhaled in high levels can cause membrane irritation, headaches, nausea and kidney irritation.
It was not immediately clear if the fire was under control.
The CSX train was traveling from Cincinnati, Ohio, to Waycross, Georgia, and consisted of two locomotives, 45 loaded rail cars carrying mixed freight and 12 empty rail cars, company spokeswoman Kristin Seay told NBC News.
"Cars of acrylonitrile are located on either side of the burning rail car," she said, adding that acrylonitrile was "flammable and presents an inhalation risk."
"CSX personnel are on hand at an outreach center that has been established. Displaced residents are being offered assistance, including lodging," she said.
Additional reporting by Chris Nelson.