Litvinenko worked for Russia's KGB security service and its successor agency, the FSB
The case of Alexander Litvinenko, the former Russian secret agent killed with radioactive poison in London in 2006, had all the hallmarks of a Cold War era thriller with a human tragedy at its centre.
A British inquiry into the death last year said Russia's President Vladimir Putin "probably approved" the killing and identified two Russians, Andrei Lugovoi and Dmitri Kovtun, as the prime suspects.
The pair, former Kremlin bodyguard Lugovoi and ex-Red Army soldier Kovtun, were the targets of US sanctions announced on Monday, along with Investigative Committee chief Alexander Bastrykin.
Here is a summary of the Litvinenko case:
Who is Litvinenko?
Litvinenko worked for Russia's KGB security service and its successor agency, the FSB, before denouncing an alleged state plot to assassinate tycoon Boris Berezovsky and fleeing from Russia to Britain in 2000.
He became a British citizen and an outspoken critic of Putin, co-operating with various secret services including Britain's MI6, as well as working for private security companies based in London.
Litvinenko also befriended exiled Chechen separatist leaders and converted to Islam. He was buried with Muslim rites at Highgate cemetery in north London in a lead-lined coffin to prevent radiation leakage.
What happened?
Litvinenko had tea with Lugovoi and Kovtun in a London hotel on November 1, 2006, for a meeting to discuss possible future business opportunities.
The inquiry found that this may have been a second poisoning following an earlier attempt when the three met for the first time in London in October.
Litvinenko felt ill immediately after the November 1 meeting and his condition steadily deteriorated.
The poison was identified as Polonium-210 -- a rare and expensive radioactive isotope produced in Russia.
His condition steadily deteriorated and he died on November 23. In a letter apparently dictated on his deathbed, he accused Putin of being behind his murder.
Radiation trails were detected in various spots around London, prompting a major security alert in the British capital after what the Litvinenko family's lawyer Ben Emmerson called "an act of nuclear terrorism".
Who are Lugovoi and Kovtun?
The pair grew up in the same apartment block as children and went to the same military college.
After the collapse of the Soviet Union, Lugovoi joined a successor state security organisation before leaving to go into business in the security sector.
In the wake of Litvinenko's death, Lugovoi became a public figure, getting elected to parliament for the pro-Kremlin Liberal Democratic Party of Russia in 2007 and even presenting a series on Russian and Soviet traitors on state television.
Much less is known about Kovtun, who is not a public figure in Russia and rarely talks to media.
In his witness statement, Kovtun said he joined the Soviet army and went abroad, serving in then Czechoslovakia and East Germany.
Kovtun went on to live in Hamburg until 2003.
What happened later?
The Litvinenko case plunged relations between Britain and Russia to their lowest levels since the Cold War, although they have since recovered.
Britain was however a major proponent of EU sanctions against Russia over the conflict in Ukraine.
Litvinenko's patron and friend Berezovsky was found dead in his mansion near London in March 2013 with one end of his favourite scarf wrapped around his neck and the other to a shower curtain rail.
A British coroner recorded an open verdict into the death, saying it was "impossible" to confirm suicide.