Over the summer, Vladimir Putin was asked about the poisoning of a former Russian double agent and his daughter last year in Britain. The Russian president denied his government was involved, but he didn't hold back on what he thought of traitors.

Treason is not only the "gravest crime possible" but also the "most despicable crime that one can imagine," he told the Financial Times. Putin, a former KGB agent when Russia was part of the Soviet Union, repeated twice to the journalists: "Traitors must be punished."

This attitude toward Russians who work with foreign intelligence services is one reason that the identity of a CIA source who provided important information about Putin and the Kremlin has been kept secret by U.S. officials.

Though the source was exfiltrated from Russia in 2017 and is in the United States, his safety cannot be guaranteed. Just a day after the news of the source's existence was confirmed in the U.S. media, Russian news outlets published the name of a former Russian official living in the D.C. area.

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Joseph Augustyn, a former director of defector resettlement operations at the CIA, said that the U.S. government would be well aware of the risks and probably would have 24-7 security on the defector. "Putin is very revengeful. Putin will go after these people," Augustyn said.

A number of Kremlin foes who have fled abroad have met unfortunate fates. Sergei Skripal, the aforementioned former Russian double agent, was found slumped on a park bench in the English town of Salisbury alongside his daughter Yulia in March 2018.

Skripal, then 66, had been jailed in Moscow for sharing the names of undercover Russian intelligence agents working overseas with European authorities. He had been living in Britain since 2010, following a prisoner exchange after the discovery of 10 Russian sleeper agents in the United States.

British police later found that Skripal had been poisoned with a deadly Soviet-era nerve agent known as Novichok. Though both the Russians survived, a British woman, Dawn Sturgess, later died of accidental contact with the nerve agent.

British authorities soon identified two Russians who they said were behind the assassination attempt on Skripal; Britain and its allies expelled scores of Russian diplomats in the wake of the attack.

Not all such attempts fail. Alexander Litvinenko, a former KGB operative who had turned into a high-profile critic of Putin, died three weeks after drinking tea laced with a radioactive substance in 2006. Ten years later, a British public inquiry found that Putin "probably" ordered the killing. The Kremlin, denying involvement, has refused to extradite the two men British authorities have accused in the attempt on Litvinenko.

Many seemingly similar events are never officially confirmed as Russian assassinations, giving an air of mystery to the deaths. There was the Russian billionaire who was found apparently hanged in his bathroom in Britain; a former Moscow businessman turned whistleblower who dropped dead of a heart attack in 2012; and perhaps more unambiguously, the Chechen exile shot dead in Berlin last month.

These may be only the tip of the iceberg. Some suspect cases have occurred within the United States, too, even within the Washington area. A onetime aide to Putin who had helped launch the Kremlin's global English-language Russia Today television network, Mikhail Lesin, was found dead in a Dupont Circle hotel in 2015.

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Though Lesin's death was ruled accidental - a result of head injuries sustained after days of "excessive" drinking, according to Washington authorities - speculation has followed it.

Calder Walton, a historian at Harvard University who studies espionage, said that it did appear there was a "red line" for Russia when it came to assassinations on U.S. soil. Though, he added, Putin, who is believed to have personally ordered election interference in the United States, could choose to step over that line.

Historically, though few Soviet defectors to the United States are believed to have been assassinated, there has always been speculation. When Walter Krivitsky, a Soviet intelligence officer who revealed plans of a neutrality pact with Nazi Germany, was found dead of gunshot wounds in 1941, many suspected foul play.

"Was it suicide? Or was he suicide-ed?" Walton said.

By refusing to admit any link, even when evidence seems to confirm Russian involvement, Putin in effect allows all suspect deaths of Russian exiles to look like retribution.

But the Russian president is never ambiguous when it comes to what he thinks of those he views as traitors. Speaking at an international energy forum in October, Putin suggested that though Russia had no reason to attack Skripal as he had already served time in prison, Putin wasn't concerned about Skripal's well being either.

"He's just a spy, a traitor to the motherland," Putin said. "Think about it as a citizen, what would be your attitude to someone who betrayed your own country? He's just a scumbag."