Three people were arrested late Wednesday in Slovakia and Hungary for trying to sell 1 kg of radioactive material.
Two of the suspects were arrested in Slovakia, and the third was arrested in Hungary in a special joint operation, Slovak police have said.
According to the Czech news agency CTK, the material was enriched uranium, supposed to be sold for USD 1M.
The Slovak authorities said specialists are still examining the substance.
Erich Tomas, a spokesman for the Slovak Interior Ministry, told reporters he had no information about the case.
In 2003, the Czech police arrested two Slovaks in the city of Brno after they allegedly sold undercover officers bars of low-enriched uranium for $715,000.
There have been long-time concerns that Eastern Europe could be a source of radioactive material for a "dirty bomb" designed to kill people by dispersing radiation, as security at nuclear-related industries deteriorated after the collapse of communist regimes in 1990.