Which radioactive isotope was used to kill Alexander Litvinenko in 2006?

Alexander Litvinenko was a former officer of the Russian Federal Security Service (FSB) and KGB. He fled from court prosecution in Russia and received Political asylum in the United Kingdom. On 1 November 2006, Litvinenko was hospitalized and died three weeks later. He was the first confirmed victim of lethal ''polonium-210'' induced acute radiation syndrome. Polonium-210 is extremely toxic, with one microgram being enough to kill the average adult. It is 250,000 times more toxic than hydrogen cyanide by weight. The 'polonium-210' is very expensive due to its minor fleeting presence in uranium-238 ores. The investigations by British authorities into the circumstances of Litvinenko's death led to serious diplomatic difficulties between the British and Russian governments. During the 2014-15 trial on Litvinenko's death, the witness by Scotland yard representative, established the credible evidences that the Russian state is involved in the Litvinenko's murder.