Uranium seized in Slovakia on Wednesday was enriched enough to be used in a so-called "dirty bomb" to spread radioactive material, police say.
The material could have been used by terrorists, police said
The uranium was found during a raid on alleged smugglers near the Hungarian border. Three people were detained.
The suspects had just under 500g (17.6 ounces) of uranium in powder, senior police official Michal Kopcik said.
Slovakian investigators believe the enriched uranium came from somewhere in the former Soviet Union.
The identity of the intended buyer for the uranium has not been released.
A dirty bomb uses conventional explosives to scatter radioactive debris and contaminate the target area.
The uranium seized on Wednesday could have been used "in various ways for terrorist attacks", Mr Kopcik said.
"The radioactive uranium was even more dangerous because it was in powder form," he said.
Mr Kopcik said the confiscated material contained uranium-235 as well as the widely occurring uranium-238.
The police did not specify the degree to which the uranium had been enriched to raise the proportion of the 235 isotope.
Enriched uranium with boosted levels of the 235 isotope can be used to make bombs or fuel power plants.
Hungarian and Slovak police are believed to have spent months tracking the alleged smugglers.
The police originally said 1kg (35 ounces) of radioactive material had been confiscated in Wednesday's raid but this figure was revised downwards on Thursday.
The raid near the Hungarian border in Slovakia coincided with the arrest of one Ukrainian citizen and two Hungarians, all aged between 40 and 51 years.
International agencies have repeatedly warned of the risk of radioactive material from the former Soviet Union passing into the hands of criminals or terrorists.
A police raid in the Czech Republic in 1994 uncovered an attempt to sell 2.73kg (96 ounces) of enriched uranium illegally.
Police in the same year confiscated 2.97kg (105 ounces) of enriched uranium intended for illegal sale in the Russian city of St Petersburg.