Two years ago, a mysterious spike in radioactivity was detected in central Europe, with particles observed in Germany, Italy, Austria, Switzerland and France.
It’s not thought to be harmful to health - but researchers now believe they know where the isotope Ruthenium-106 came from.
A study has traced the cloud back to a site in the southern Urals, which is where Russia's Majak nuclear facility is located.
It's likely the material leaked from a reprocessing planet, although its exact origin is 'difficult to determine'.
A total of 70 experts contributed to the report, which was published in the Proceedings of the National Academy of Sciences of the USA (PNAS) journal.
Professor Georg Steinhauser from the University of Hanover said: 'We measured radioactive ruthenium-106.
'The measurements indicate the largest singular release of radioactivity from a civilian reprocessing plant."
In autumn 2017, a cloud of ruthenium-106 was measured in 'many European countries' with a maximum radiation level of 176 millibecquerels per cubic meter of air, which were up to 100 times higher than the total level measured in Europe after the Fukushima incident.
Even though it was the most serious release of radioactive material since Fukushima in 2011, the public 'took little notice of it' and Russia has not admitted responsibility,