A mysterious radiation cloud spread over Europe in September. Russia finally acknowledged it.

After weeks of denying its existence, the Russian government this week acknowledged the strange surge of radiation that billowed over Europe in September.

The French nuclear safety regulator IRSN first detected the radioactive element ruthenium 106 in the air in late September, tracing its origins to the Ural Mountains in the border region between Russia and Kazakhstan. Other European cities like Stockholm, Milan, and Budapest also began picking up radiation traces.

Ruthenium is a rare transition metal. The ruthenium 106 isotope has a half-life of 373 days and is used to treat cancers like melanoma. Since it doesn’t occur in nature, any measured ruthenium 106 is assumed to originate from human activity.

It’s also a byproduct of splitting uranium 235 in nuclear reactors — a hint of the cloud’s possible origin.

IRSN, which said the radiation was no longer detectable as of October 13, concluded that it was an accidental radiation leak and estimated that the amount of radiation released at the source was somewhere between 100 and 300 terabecquerels. That’s enough to require shielding precautions and special cleanup procedures in the region near the origin, but was not a threat to human health farther away. (A terabecquerel is equal to a trillion nuclear disintegrations per second, a measurement of the radioactivity of a substance.)

Patrick Regan, a professor nuclear physics at the University of Surrey in the United Kingdom, told the Science Media Centre that the radiation most likely originates from a nuclear waste reprocessing plant.

“If it was a reactor leak or nuclear explosion, other radioisotopes would also be present in the ‘plume’ and from the reports, they are not,” he said.

German nuclear observers also ruled out a nuclear power plant accident as the source of the radiation.

For their part, Russian officials denied detecting any leaks or elevated radiation at any of their facilities. This was very odd given that 43 nearby countries reported elevated levels of ruthenium 106.

But finally, in response to an information request from Greenpeace, the Russian meteorological agency Roshydromet released a report this week that acknowledged measurements of high ruthenium 106 levels.

Russian authorities said atmospheric radiation levels were higher outside of Russia, indicating that the source of the leak wasn’t inside the country. However, the Institute of Nuclear Physics in Kazakhstan reported no radiation leaks from any of its facilities.

“The published data is not sufficient to establish the location of the pollution source,” Maxim Yakovenko, the head of Roshydromet, said in a statement to the New York Times.

Greenpeace Russia said they are now asking the public prosecutor to investigate a possible cover-up of a radiation leak.

Russian officials did report high radiation levels at two monitoring stations near the Mayak nuclear facility in the Chelyabinsk region. The site was used to make fuel for nuclear weapons and nuclear reactors, and in 1957, suffered a massive explosion that created the worst nuclear disaster in history until Chernobyl.

The Mayak facility stopped making plutonium for nuclear weapons in 1987, but still reprocesses spent nuclear fuel from reactors. It’s also famous because U2 spy plane pilot Francis Gary Powers was shot down in 1960 while attempting to surveil it.

Still, we don’t know where this radiation came from, and other reports suggest that the Research Institute of Atomic Reactors in Dimitrovgrad, Russia — 90 miles north of Mayak — is another possible source. Given Russia’s notoriously opaque nuclear sector, we might be waiting awhile.

Correction: An earlier version of this article said a terabecquerel is equal to a billion nuclear disintegrations per second, when it is actually equal to a trillion.