A mysterious radioactive cloud hovering above Europe came from Russian nuclear plant pollution, the nation admitted Monday, according to the South China Morning Post. Russia’s meteorological service confirmed there were “extremely high” concentrations of radioactive isotopes in the air in September.

The cloud was first discovered over Italy in early October. Authorities said at the time it appeared the radioactivity originated somewhere in South Russia between the Volga River and the Ural Mountains, according to Jean-Christope Gariel of France’s Radioprotection and Nuclear Safety Institute.

“Probes of radioactive aerosols from monitoring stations Argayash and Novogorny were found to contain radioisotope Ru-106 [between Sept. 25 and Oct. 1],” Russia's Rosgidromet service said, according to the South China Morning Post.

Authorities said the highest concentration was found in Argayash. Though the specific location of the pollution’s source was not given, Argayash lies near the Mayak nuclear facility. The Mayak plant has had a number of nuclear accidents in the past, including one in 1957 that was the second biggest nuclear disaster in history. A storage tank with highly radioactive liquid waste exploded at the time, releasing more than half the amount that was released during the Chernobyl disaster.

Mayak, which currently serves as a reprocessing site for nuclear fuel, ranks as one of the most radioactive places on earth, according to Greenpeace. The plant, however, denied any involvement in the release of the radioactive material, Russian news outlet the Medusa Project reported. 

"At the Mayak plant in 2017, sources from ruthenium-106 were not produced, the air emissions were in the usual regulatory values," a statement from Mayak said.

It remained unclear how the recent radioactive cloud of pollution got into the air, as it was not an “authorized release” and there was no evidence of an accident, authorities said. Radioactivity was located in the air in Germany, Italy, Austria, Switzerland and France, but officials said the quantity posed no threat to human health.  The quantity released at the pollution source, however, may have been enough to require taking shelter or evacuating.

“If it would have happened in France, we would have taken measures to protect the population in a radius of a few kilometers,” Gariel said.

Russia Mayak Nuclear Plant A view of the village of Novomuslyumovo, built under a relocation program 1 mile away from the old village of Muslyumovo, Nov. 17, 2010. Muslyumovo is located on the banks of the Techa river in Russia's Urals, one of the country's most lethal nuclear dumping grounds. The Mayak nuclear complex located 18 miles from Muslyumovo, currently processing foreign radioactive wastes, dumped 2.68 billion cubic feet of highly radioactive waste into the river from 1949 to 1956. Photo: Reuters