A map from French authorities suggests that the release came from the east, near the border of Russia and Kazakhstan.

European authorities are providing new details about a cloud of mysterious radioactive material that appeared over the continent last month.
Monitors in Italy were among first to detect the radioactive isotope ruthenium-106 on Oct. 3, according to a fresh report by France’s Radioprotection and Nuclear Safety Institute, known as IRSN. In total, 28 European countries saw the radioactive cloud, the report says.
The multinational Comprehensive Nuclear-Test-Ban Treaty Organisation, which runs a network designed to monitor for nuclear weapons tests, also confirmed to NPR that it had detected the cloud.
Based on the detection from monitoring stations and meteorological data, the mysterious cloud — which has since dissipated — has been traced to somewhere along the Russia-Kazakhstan border, according to Jean-Christophe Gariel, director for health at the IRSN.
“It’s somewhere in South Russia,” he says, likely between the Volga River and the Ural Mountains.
Authorities say the amount of material seen in Europe was small. “It’s …
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