Russian investigators on Wednesday detained three staff at a power plant where thousands of tons of diesel leaked into the soil and waterways of the Arctic region.

The spill of over 21,000 tons of fuel, which environmentalists say is the largest ever in the Arctic, took place after a fuel reservoir collapsed at a power plant operated by a subsidiary of metals giant Norilsk Nickel in the city of Norilsk beyond the Arctic Circle.

President Vladimir Putin declared an emergency situation and Norilsk Nickel head Vladimir Potanin told the president the company would pay for clean-up efforts estimated at $146 million.

The Investigative Committee, which is probing the accident, said it had detained the director of the power station, Pavel Smirnov, along with two engineers on suspicion of breaching environmental protection rules. If charged, they would risk up to five years in prison.

The Investigative Committee said the fuel tank had required major repairs from 2018 but the suspects “continued to use it in breach of safety rules.” 

“As a result the accident occurred,” the investigators’ statement said.

In materials distributed Tuesday, Norilsk Nickel said that the fuel reservoir was built in 1985 and underwent repairs in 2017 and 2018 after which it went through a safety audit.

“The company considers this measure to be unjustifiably harsh,” the press service said in a statement to the state-run RIA Novosti news agency, citing vice president Nikolai Utkin.

He said those detained “are cooperating with law enforcement authorities and now they would be much more useful at the scene of the clean-up operation.”

Regional officials have said that despite efforts to contain the fuel leak using booms on the surface of a river, it has now reached a freshwater lake that is a major source of water for the region.

The pollution could now flow into the Kara Sea in the Arctic Ocean north of Siberia, which Greenpeace Russia expert Vladimir Chuprov told AFP would be a “disaster.”

Norilsk Nickel’s first vice president denied the spill had reached the lake on a conference call on Wednesday, saying the company did not find contamination there.

The metals giant has said the accident could have been caused by global warming thawing the permafrost under the fuel reservoir. 

The company has acknowledged it did not specifically monitor the condition of permafrost at its sites.

Its board of directors is due to meet Wednesday to discuss the accident, the company’s press service confirmed to AFP.