In late September, the European atmospheric networks from the south of France north to Germany and Denmark began detecting tiny amounts of a strange radioactive substance in the air. Since it was only that isotope that was being released in their air, they quickly ruled out release from a nuclear reactor – and almost as quickly ruled out a fallen satellite, since there were none. Eventually two of the agencies tracked the radioactivity back to the southern Ural Mountains, in Russia.

Now a group of observers including the Russian meteorologists are pointing to a nuclear production plant near the country’s border with Kazakhstan – even as the state-run agency denies it is behind the release.

The Mayak Production Association is the facility, according to Greenpeace Russia. The Russian agency that runs the operation denies it.

“The atmospheric pollution by the isotope Ruthenium-106, indicated in the report of Roshyrdromet (the Russian meteorology office), is not related to the activities of the FSUE Mayak Production Association,” said Rosatom, the state agency running the site, in a statement today.

“Taking into account Roshydromet’s recent statements the state corporation Rosatom should thoroughly investigate and publish data on the events at the Mayak Chemical Combine or at other enterprises,” said Greenpeace Russia, in a statement Monday, citing figures from Russian sources as well as the International Atomic Energy Agency. “Greenpeace Russia will send a letter to the prosecutor’s office with a request to verify the possible concealment of a radiation accident and information on the state of the environment, as well as the readiness of a system for monitoring radionuclides in the atmosphere to new accidents.”

The Ruthenium-106 is not found in the natural world, and is only made synthetically through byproducts of other radioactive chemical endeavors.

It has been a marker for other secretive mishaps by Russia, most notably the Chernobyl disaster in 1986. However, the radioactive plume that was detected over the rest of Europe as it made its way from the Ukraine to the west was marked with other isotopes. (Soviet Union officials denied the clear trail of radioactivity back to the destroyed Chernobyl plant, until the Swedish government told the Soviet government it was preparing a report for the IAEA, which prompted them to release a short statement on state TV).

The French Institut de Radioprotection et Surete Nucleaire (IRSN) concluded earlier this month that Ruthenium-106 alone eliminates the possibility of nuclear reactor releases.

“ISRN has, in its investigations, made the hypothesis of a rejection from an installation,” they conclude.

The Bundesamt fur Strahlenschutz, the German Federal Office for Radiation Protection, concurred – and pointed toward Russia.

“Analysis of the source of the radioactive material is highly likely to indicate release in the southern Urals, but other regions in southern Russian cannot be excluded,” they concluded.

The Mayak facility, which was constructed right after World War II in the Soviet Union’s quickly-developing nuclear weapons program, was host to the Kyshtum disaster of 1957, which resulted in hundreds of cases of cancer – and which is listed by some as the third most serious nuclear accident in history (after Chernobyl and the Fukushima Daiichi disaster of 2011).