The European Court of Human Rights recently backed a UK inquiry that said Russia was responsible for the killing.
Fifteen years after the death of Alexander Litvinenko, his family is still seeking justice.
The former KGB spy died from poisoning in London on 23 November 2006 after drinking tea laced with radioactive polonium-210.
In a deathbed statement, Litvinenko placed the blame on Russian President Vladimir Putin. Moscow has always denied the accusations.
In September, the European Court of Human Rights backed British authorities and stated that Russia was responsible for the killing.
Litvinenko, a former agent of the KGB and then Russian Federal Security Service (FSB) defected from Russia in 2000 and fled to London.
While in the UK, he became involved in exposing corruption and links to organised crime within Russian intelligence.
He fell suddenly and violently ill on 1 November 2006, after meeting two Russian men at a hotel, and spent three weeks in the hospital before he died.
In 2016, a UK inquiry concluded that President Putin had "probably approved" the operation by two Russian agents to kill Litvinenko.
Widow Marina has continually vowed justice for her husband, even after the European court ruled she would receive damages and costs worth €122,500 from Russia.