A cloud of radioactivity detected over Europe during a period of several weeks could have resulted from an accident at a nuclear facility in Russia or Kazakhstan in the last week of September, the IRSN nuclear safety institute in France has said.

The revelation focuses the mystery surrounding high levels of ruthenium 106 detected by numerous European monitoring stations throughout October, but officials in neither Russia not Kazakhstan have yet indicated that any accidents have occurred on their territories.

Although IRSN – the technical arm of ASN, the French nuclear regulator – could not determine the exact location of the radioactive release, it said that an analysis of weather patterns suggests it came from somewhere between the Volga River and the Ural mountains.

This puts the location of the release somewhere in Russia or Kazakhstan, Jean-Marc Peres, IRSN’s director, told Reuters.

Though the release currently poses no danger, the institute said it should have triggered local evacuations and other civil defense measures for several kilometers in the area where it occurred.

But neither Bellona nor IRSN is aware of any such events during the past several weeks within Russia. IRSN’s Peres told Reuters his organization had yet to contact Kazakh authorities to establish if an emergency response had taken place there.

“As far as we know there has been no known local protective measures for the population in either Russia or Kazakhstan, which means that the authorities in the country responsible for the leak have chosen to keep quite about it,” said Nils Bøhmer, Bellona’s general manager and nuclear physicist.

He called on the International Atomic Energy Agency to investigate the contamination.

A spokeswoman for the Russian Emergencies Ministry told Reuters she could not immediately comment on the release. Reuters was unable to obtain comment from authorities in Kazakhstan or the Kazakh embassy in Moscow.

In recent weeks IRSN and several other nuclear safety institutes in Europe had measured high levels of levels of ruthenium 106, a radioactive nuclide that is the product of splitting atoms in a nuclear reactor and which does not occur naturally.

IRSN said in its statement that the quantity of ruthenium 106 released was major – between 100 and 300 teraBecquerels. If an accident of that magnitude had occurred in France, IRSN said, it would have triggered evacuations for several kilometers around its source.

Peres told Reuters the nuclide likely came from a nuclear fuel treatment site, or an institute that deals with nuclear medicine.

Yet IRSN told Reuters it has ruled out an accident in a nuclear reactor, as that would have led to contamination by additional radionuclides.

Measurement from European stations showed high levels of ruthenium 106 in their atmospheres beginning in early October, and steadily decreasing from October 6th.

The IRSN said that the concentrations of ruthenium 106 in the air that have been recorded in Europe were of no consequence for human health or the environment.

Small, inexplicable releases of radiation are not uncommon. In January, trace amounts of radioactive iodine were detected over several European countries, the origins of which remain unclear.