Random and mysterious radioactive particles were found across Europe last week and experts are still struggling to find the cause. The slight increase in radiation was detected by several trace measuring stations in Europe, including at least six in Germany.

Ruthenium-106 is used in medicine to treat tumors less than 7 millimeters in depth and eye cancers, as well as in energy where it is can be used to power satellites.

Levels of the radioactive isotope were found at elevated – but not harmful – levels in Germany, Italy, Austria, Switzerland, and France between September 29 and October 3, according to Germany’s Federal Office for Radiation Protection.

In Germany, ruthenium-106 was measured at values between a few microbequelel and 5 microbequelel per cubic meter of air – so as unusual as it was, it didn't pose a danger to public health.

What caused the radioactivity spike remains a bit of a mystery but a spokesperson from the office has been able to confirm that it originated in Eastern Europe. While they haven't been able to nail down the precise location, they think it probably came from the Ural, a federal district in Russia. Officials haven't ruled out other locations in Southern Russia.

They can, however, rule out the possibility of an accident at a nuclear power plant as a possible cause. If this had been the case, ruthenium-106 wouldn't be the only radioactive substance to have increased in volume.

Something very similar happened in January and February this year but with the radioactive substance iodine-131 instead of ruthenium-106. The French Institute of Radioprotection and Nuclear Safety (IRSN) reported spikes in levels of the isotope across Europe, from Spain to Finland. Again, they pinpointed its origin to Eastern Europe.

While there were some exciting – if terrifying – rumors that Russia had secretly tested a low-nuclear weapon in the Arctic resulting in nuclear fallout, the reality was far more mundane. Experts pointed out that if there had been a nuclear test, the control stations would have detected elevated levels of more than one type of radioactive chemical. Like ruthenium-106, iodine-131 is used for medical purposes and the increase most likely came from a drug company producing radioactive pharmaceuticals.