FILE - In this file photo taken on Friday April 8, 2016, a sign warns people not to enter the town of Ozersk, Chelyabinsk region, Russia, which houses the Mayak nuclear facility. Mayak is a nuclear complex that has been responsible for at least two of the country's biggest radioactive accidents. Russian authorities denied Friday that a radioactivity spike in the air over Europe resulted from a nuclear fuel plant leak in the Urals, saying their probe has found no release of radioactivity there. (AP Photo/Katherine Jacobsen, File)

FILE - In this file photo taken on Friday April 8, 2016, a sign warns people not to enter the town of Ozersk, Chelyabinsk region, Russia, which houses the Mayak nuclear facility. Mayak is a nuclear complex that has been responsible for at least two of the country's biggest radioactive accidents. Russian authorities denied Friday that a radioactivity spike in the air over Europe resulted from a nuclear fuel plant leak in the Urals, saying their probe has found no release of radioactivity there. (AP Photo/Katherine Jacobsen, File)

MOSCOW (AP) — Russian authorities denied Friday that a radioactivity spike in the air over Europe this fall resulted from a nuclear fuel processing plant leak in the Ural mountains, saying their probe has found no release of radioactivity there.

Vladimir Boltunov of Russia’s Rosatom state nuclear corporation said an inspection of the Mayak nuclear plant has proven that it wasn’t the source of Ruthenium-106, a radioactive isotope spotted in the air over Europe and Russia in late September and early October.

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France’s nuclear safety agency said last month that increased levels of Ruthenium-106 were recorded over most of Europe but posed no health or environmental risks.

The Russian panel that involved experts from Rosatom and other agencies failed to identify where the isotope came from, but alleged it could have come from a satellite that came down from its orbit and disintegrated in the atmosphere.

Nuclear safety expert Rafael Arutyunian said while isotopes of plutonium, cesium or strontium are normally used as power sources for satellites, it can’t be excluded that Ruthenium-106 could have been used in some satellite equipment.

The assumption that the isotope came from a crashing satellite would explain its broad spread over Europe, he argued.

Arutyunian, deputy head of the Institute for Safe Nuclear Energy of the Russian Academy of Sciences, said that a broader panel will continue investigating the radioactivity.

Last month, the Russian state meteorological office reported high levels of Ruthenium-106 in late September in areas close to Mayak, but Arutyunian and other experts emphasized that they were still tens of thousand times less than the level that would pose health risks.

The environmental group Greenpeace alleged that Mayak could have been the source of a Ruthenium-106 leak, but the panel insisted the plant doesn’t extract the isotope or conduct any other operations that may lead to its release.

The commission said a thorough inspection of the plant had found no safety breaches and checks of its personnel also hadn’t detected any trace of the isotope.

Vyacheslav Usoltsev of Rosatom’s safety inspectorate said a sophisticated monitoring system at the plant would have spotted any release of radiation.

The panel also noted that while increased levels of Ruthenium-106 were spotted in the Urals and over Europe, they weren’t detected over a 2,000-kilometer (1,250-mile) swath of land between the Urals and Russia’s western border. It argued that if the source of the leak were on the ground, it would have spread the trace of Ruthenium-106 midway.

Mayak, in the Chelyabinsk region, saw one of the world’s worst nuclear accidents on Sept. 29, 1957, when a waste tank exploded. That contaminated 23,000 square kilometers (9,200 square miles) and prompted authorities to evacuate 10,000 residents from neighboring regions.