Just weeks after Russia’s nuclear energy corporation said radiation levels were normal, the country’s meteorological service said it had registered extreme levels of radiation in the Urals two months ago.

European radiation monitors detected a cloud of radioactive material originating in Russia’s southern Urals over two dozen European countries in late September and early October. The material could have come from “a spent nuclear fuel reprocessing or the production of radioactive sources."

Russia’s state nuclear corporation Rosatom said the release of the radioactive material, known as Ruthenium 106 or Ru-106, was natural background radiation, the state-run RIA Novosti news agency reported at the time.

But on Monday, Russia’s Federal Meteorological Service said it had registered extreme levels of Ru-106 across several locations in late September. The highest levels of the radioisotope — 986 times the norm — were located in Chelyabinsk region, the site of the Mayak nuclear facility.

Mayak makes components for nuclear weapons and processes spent nuclear fuel. One of its storage facilities exploded in 1957 — a fact that went unreported until three decades later.

In a statement issued on Tuesday, Mayak said it was not the source of the excess Ru-106 concentration.

