A British inquiry found that Russian President Vladimir Putin "probably" signed off on the poisoning of Alexander Litvinenko in 2006 in London.
The report said two Russians, Andrei Lugovoi and Dmitry Kovtun, put the radioactive substance polonium-210 in Litvinenko's drink at a hotel, which led to his hospitalization and death.
Possible motives included his work for British intelligence agencies, criticism of Moscow's FSB intelligence service and connection to other Russian dissidents, the BBC reported.
Inquiry Chairman Sir Robert Owen said the evidence heard in court amounted to a "strong circumstantial case" that the Russian state was behind the death, reported the Guardian.
The Russian Foreign Ministry called the public inquiry "politicized."
Litvinenko, who became a whistleblower of Moscow's security services, fled Russia to Britain with his wife and son in 2000.