NORILSK, RUSSIA (NBC) — A small river in the northern part of Siberia turned bright red, raising concerns of local citizens and infuriating environmental activists.

Residents of the Siberian town of Norilsk reported that Daldykan River, outside the city, changed in color on Sept. 6.

Russia’s Ministry of Natural Resources and Environment said in a statement that an investigation was underway, adding that a possible cause was a breakdown of the facilities of the Norilsk Nickel Company.

But Norilsk Nickel rep representatives denied the information about the breakdown.

Greenpeace Russia provided pictures from the site, showing river banks covered in a dark red slime.

The head of the organization’s energy program, Vladimir Chuprov, said Norilsk Nickel bears responsibility for the disaster.

The factory is the world’s second-largest nickel producer. The company is also the world’s largest producer of palladium.

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