Thai police, tipped off by US investigators, have arrested a man selling radioactive material that can be used to make a so-called dirty bomb.
They did not say if the suspect was believed to have links to terrorists, but the arrest comes at a time of heightened tension in Thailand.
Earlier this week police said they had foiled a plot to bomb embassies in the capital.
Officers arrested Mr Narong Penanam (44), in the car park of a Bangkok hotel after he offered to sell agents a metal container that he said held uranium, police Col Pisit Pisutisak said.
He expected to be paid around €250,000.
An analysis of the material by the Office of the Atomic Energy for Peace later revealed it was not uranium but caesium-137, which has a number of medical and industrial applications and can used in a bomb designed to spread radioactive material over a wide area.
Mr Narong said he got it from neighbouring Laos. He was charged with illegal possession of radioactive materials, punishable by up to a year in prison and a fine of €250.