Someone is hiding a nuclear accident in Europe or Central Asia. French nuclear safety experts say the release happened somewhere in Russia or Kazakhstan in the last week of September.

The French nuclear safety institute IRSN said the radioactive cloud most likely came from a nuclear fuel treatment site or centre for radioactive medicine, due to the specific nature of the material, a mixture not found in a general nuclear power plant explosion.

The French regulator said the accident happened south of the Ural mountains, between the Urals and the Volga river.

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“Russian authorities have said they are not aware of an accident on their territory,” IRSN director Jean-Marc Peres told Reuters. He added that the institute had not yet been in contact with Kazakh authorities, reported Reuters.

Officials have detected a high level of ruthenium 106, a radioactive nuclide that is the product of splitting atoms in a nuclear reactor and which does not occur naturally, reported Reuters.

Some experts also have reported the release could have come from Ukraine, due to technical management problems at their nuclear facilities and general corruption.

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The airborne material was detected for the first time in early October.

Another possibility could be a returning satellite but there have not been any recently returned to Earth that carried the material.

Soviet officials did not announce the Chernobyl disaster until days after the explosion which sent a radioactive plum across Europe. Chernobyl is now located in northwestern Ukraine.