On 1 November 2006, Litvinenko suddenly fell ill and was hospitalised in what was established as a case of poisoning by polonium-210; he died from the poisoning on 23 November. He became the first known victim of lethal polonium 210-induced acute radiation syndrome. The events leading up to this are a matter of controversy, spawning numerous theories relating to his poisoning and death. A British murder investigation pointed to Andrey Lugovoy, a former member of Russia's Federal Protective Service (FSO), as the prime suspect. Dmitry Kovtun was later named as a second suspect. The United Kingdom demanded that Lugovoy be extradited, however Russia denied the extradition as the Russian constitution prohibits the extradition of Russian citizens, leading to a straining of relations between Russia and the United Kingdom.