A radioactive cloud of pollution that spread across Europe in September and October likely originated in Russia, or possibly Kazakhstan, French nuclear authorities believe.

The potential nuclear accident that launched the cloud most likely took place between the Volga river and the Ural Mountains in the last week of September, IRSN said.

Ruthenium-106 particles were detected by several countries in concentrations not harmful to human health, according to the agency, but disappeared from France in mid-October.

Jean-Marc Peres, IRSN’s director, told Reuters: “Russian authorities have said they are not aware of an accident on their territory,” adding that his team had not yet contacted Kazakh authorities.

Germany’s Federal Office for Radiation Protection said an accident at a nuclear power plant could be “ruled out” due to the type of particle detected.

It said: “Ruthenium-106 is used as a radiation source in cancer therapy for the treatment of ocular tumours. Ruthenium can also occur during the reprocessing of nuclear fuel elements.”