A British inquiry has concluded that the Russian government was behind the 2006 poisoning death of former Russian security agent Aleksandr Litvinenko and that President Vladimir Putin "probably approved" the killing.
The findings, issued on January 21, said that there was a "strong probability" that it was carried out by Russian citizens Dmitry Kovtun and Andrei Lugovoi acting under orders from the Federal Security Service (FSB), Russia's main spy agency.
British investigators concluded that Litvinenko, 44, had ingested highly radioactive polonium-210 while drinking tea in a luxury London hotel with Kovtun and Lugovoi. He died in a London hospital three weeks later, on November 23, 2006.
Traces of polonium were found at sites across London where Kovtun and Lugovoi had been, including offices, hotels, and a soccer stadium, as well as passenger jets.
Polonium-210 is an extremely expensive substance that is only produced in a handful of top-security nuclear facilities around the world.
Litvinenko’s death, and the resulting investigation, sent relations between London and Moscow to lows not seen since the Cold War and damaged Putin’s reputation in the West.
WATCH: The Litvinenko Saga
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British Inquiry Points At Putin In Litvinenko's Killing
"What happened was absolutely appalling and this report confirms what we've always believed...which is it was state-sponsored action," British Prime Minister David Cameron told reporters on January 21 in Davos, Switzerland, where he was attending the World Economic Forum.
Russia's Foreign Ministry, meanwhile, called the British inquiry "opaque" and "politically motivated," while Kremlin spokesman Dmitry Peskov mocked the report.
"More likely it can be attributed to fine British humor," he told reporters in Moscow.
The report was a "quasi-inquiry" that would only "add more poison to the atmosphere of our bilateral ties," he said.
ALSO READ: Russian Media Shrugs Off Litvinenko Report
In Washington, White House spokesman Josh Earnest suggested Litvinenko’s death was part of a pattern of oppression against Kremlin critics and political opponents, and that the government was willing to target those individuals outside the country’s borders.
"Inside Russia we see critics and political opponents of the Russian government are often subject to threats, intimidation, and in some cases, death," he said.
"This willingness to flout basic conventions around human rights and free speech and the ability to speak out and make your political views known even if they are critical of the sitting government is something that we’ve long been concerned about inside of Russia," Earnest said.
Earnest also signaled that the U.S. administration was considering a direct response to the report's findings, though he refused to give details or timing.
It was unclear how far the Cameron government was prepared to go to punish Russia. Home Secretary Theresa May announced an asset freeze for Kovtun and Lugovoi, though it was unclear if either had any assets there at all.
London has demanded their extradition, something Moscow has repeatedly refused.
The report's conclusions describe a "blatant and unacceptable breach of international law and civilized behavior," May said.
Russia's ambassador was summoned to hear London's complaints about Moscow's lack of cooperation with the investigation, though Aleksandr Yakovenko later issued a statement echoing Moscow's warnings about potential damage to bilateral relations.
WATCH: British Judge Accuses Putin In Killing Of Litvinenko
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British Judge Names Putin In Litvinenko Killing
Litvinenko had worked for the Federal Security Service (FSB) -- the main successor agency to the KGB -- before moving to Britain in 2000 and becoming an outspoken critic of Putin.
His widow, Marina, told the inquiry that her husband had been a loyal Russian agent who grew disillusioned with Russia's 1990s war in Chechnya. He was also disillusioned by what he saw as corruption within the agency, she said.
At the time of his death, Litvinenko was reportedly working for MI6, Britain's foreign intelligence service.
Marina Litvinenko said the report vindicated her husband's deathbed assertion that Putin ordered his killing.
The British government should impose "targeted economic sanctions and travel bans against named individuals...including Mr. Putin," she said.
Russian citizens Dmitry Kovtun (left) and Andrei Lugovoi drank tea with Aleksandr Litvinenko (right) shortly before he fell ill.
Russian news agencies quoted Lugovoi, a former KGB bodyguard who is now a member of the Russian State Duma, the lower house of parliament, as saying the accusations against him are "pathetic" and "nonsense.".
Kovtun also denied any involvement and said he was awaiting the conclusions of a Russian investigation.
"It seems to me that Russia's investigation knows more than the British one," Kovtun was quoted as saying by the state-run news agency TASS.
The report, authored by British Judge Robert Owen, also named Nikolai Patrushev, who was FSB director from 1999 to 2008, as having "probably approved" the killing.
Patrushev is now secretary of Putin’s Security Council and a close Putin ally.
Aleksandr Litvinenko is pictured in the intensive care unit of University College Hospital in London on November 20, 2006.
Aleksandr Goldfarb, а Litvinenko colleague who lives in London, told RFE/RL's Russian Service that the Owen probe was exhaustive and suggested that London should sanction any person or entity connected with the killing.
"As for its objectivity, the British system is set up in such a way that it is practically impossible to imagine any political pressure on the court," Goldfarb said, "even though the fact that the British government did not want this investigation and did everything to block it has been well-reported."
Goldfarb added that British sanctions over the matter could well be expanded.
"[London] could introduce sanctions against anyone who, in the court's view, was connected to this murder," he said. "For example, if the [polonium-210] was produced at a facility run by [Russian state nuclear agency] Rosatom, then its leadership or the nuclear center at Sarov, the former Arzamas-16, must be included in sanctions."
The EU and United States have imposed sanctions on Russia and on members of Putin’s circle in response to Russia’s interference in Ukraine.