Dmitry Kovtun, one of the two Russian men accused of murder of former spy and Kremlin critic Alexander Litvinenko in London, died of Covid in a Moscow hospital on Saturday.
Litvinenko died in 2006, weeks after drinking tea laced with radioactive isotope polonium 210 at a London hotel, where he met Kovtun and the other suspect, Andrei Lugovoi. The case has weighed on relations between Britain and Russia.
After Litvinenko's death, detectives found polonium in all hotel rooms where Kovtun and Lugovoi had stayed in London, as well as on Lugovoi's plane seat from Moscow and in numerous other locations including Arsenal's Emirates stadium.
The death of 56 Kovtun was first reported by Lugovoi, a former KGB bodyguard who is now a Russian MP who wrote on Saturday: This is an irreplaceable and difficult loss for us. My close and faithful friend suddenly died yesterday, as a result of a serious illness associated with a coronaviruses, Lugovoi said.
A British inquiry in 2016 concluded that the murder of Litvinenko was an operation of Russia's FSB spy agency and that the assassination was probably approved by Russian president Vladimir Putin at the time.
One of the key findings of the British inquiry led by Sir Robert Owen was phone records showing Kovtun making a phone call to another FSB colleague saying he was looking for a cook to put a very expensive poison in Litvinenko's food or drink.
Last year, the European Court of Human Rights ruled that Russia was responsible for the killing of Litvinenko, ordering Moscow to pay 100,000 85,000 in non-pecuniary damages to his widow, Marina.
In a deathbed statement, Litvinenko accused Putin of being behind his killing.
The Kremlin has denied the charges and refused to extradite the two suspects to trial.
Before the fall of the Soviet Union, Kovtun is believed to have served in the communist east at the KGB's ninth directorate, which was charged with the protection of top Kremlin officials. After the annihilation of the Soviet Union, Kovtun and his then German wife, Inna Hohne, moved to Hamburg and claimed political asylum, with him working as a waiter. He left Germany and moved to Russia where he was involved in various businesses.
In contrast to Lugovoi, who became a prominent politician after the poisoning of Litvinenko, Kovtun kept a low profile in Russia and not much is known about his private life.
Hohne previously told German detectives during the inquiry into Litvinenko's death that Kovtun was a heavy drinker who shifted between badly paid jobs and dreamed of being a porn star.
Litvinenko, who was sacked by the FSB after publicly criticising the security service's connection to organized crime, is believed to have been killed over his work for the British intelligence agencies and his claims that the FSB was responsible for the bombing of apartment blocks in Moscow and two other cities in 1999.
The Litvinenko killing was followed by a series of other poisonings of Kremlin critics that the west blamed Russia, including the attempted poisoning of former double agent Sergei Skripal in Britain in 2018 and opposition leader Alexei Navalny in Siberia in 2020.