The freight train that derailed as a result of the collapse of the Yellowstone River bridge in Montana on Saturday, was carrying hot asphalt and molten sulfur that was spilled into the waters below.

The incident took place around 6 a.m. near the town of Columbus, Stillwater County Disaster and Emergency Services said. Photos from the scene show some sort of a yellow-colored liquid spilling from the three rail cars that derailed. According to Montana Rail Link said in a statement, both the substances carried by the rail cars rapidly become solid when exposed to cooler temperatures.

Sulfur is a common element used as a fertilizer as well as an insecticide, fungicide, and rodenticide, the U.S. Environmental Protection Agency said.

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There was no immediate danger for the crews working at the site to assess the damage and remove the debris, David Stamey, the county’s chief of emergency services, said. The river is supposed to dilute the hazardous material. 

Railroad Administration officials were working at the Yellowstone River Valley with local authorities at the scene. It is a sparsely populated area mostly surrounded by ranch and farmland. The famous Yellowstone National Park is about 110 miles southwest of the area.

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Following the hazardous spill, officials took precautionary measures, shutting down drinking water intakes downstream and diverting the flow of river water into an irrigation ditch. Initial air quality tests confirmed that sodium hydrogen sulfate carried by two of the cars was not released, the rail company said. The bridge’s structural integrity and its maintenance records, repair, and inspections are being investigated at the moment to determine the cause of its collapse.

Officials were also developing “appropriate cleanup, removal and restoration efforts” after assessing the impact on natural resources, the rail company said.