Police in Slovakia and Hungary have arrested three people for allegedly trying to sell 1kg (2.2lb) of radioactive material, officials said.
Slovak police spokesman Martin Korch said the three were planning to sell the unspecified material in Slovakia for $1m (£483,000).
He said Slovak and Hungarian police had been working together on the case for several months.
Two suspects were picked up in Slovakia and one was arrested in Hungary.
Mr Korch said experts were examining the radioactive material to determine exactly what it was.
He said the arrests had been made in the eastern part of Slovakia and Hungary near their borders with Ukraine.