Michal Kopcik, the vice-president of the Slovak police, said the suspects, who were arrested on Wednesday in eastern Slovakia and Hungary, had nearly 500g of uranium in powder form that investigators believe came from an unspecified former Soviet republic.
"It was possible to use it in various ways for terrorist attacks," Mr Kopcik said. Investigators are still trying to determine who was trying to buy the uranium, which the trio allegedly was selling for about 500,000.
Mr Kopcik said police had intelligence suggesting that the suspects - whose names were not released, but who are all men aged 40, 49 and 51 - had planned to close the deal between last Sunday and Wednesday. Police moved in when the sale failed to materialise.
Tests showed the powder contained 98.6 per cent uranium-235. It is considered weapons-grade if it contains at least 85 per cent uranium-235. The arrests heightened long-standing concerns that eastern Europe is serving as a source of radioactive material for a "dirty bomb", which would use conventional explosives to scatter radioactive debris.
About 25kg of highly enriched uranium or plutonium is needed to fashion a crude nuclear device. But a fraction of that is enough for a dirty bomb - a weapon whose main purpose would be to create fear and chaos, not casualties in the main.