The former security officer became an outspoken critic of the Russian government and fled in 2000 to London, where he was being paid by British intelligence services. Russia allegedly took revenge in a manner befitting a Cold War spy novel: poisoning Litvinenko by putting a massive dose of a rare nuclear isotope, polonium, into his teacup.

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Lying in his hospital bed shortly before his death, Litvinenko had blamed Vladimir Putin for his assassination. In January, an inquiry by a British judge concluded that the Russian president may have been involved in the slaying. Former British Prime Minister David Cameron condemned Litvinenko's death as a “state-sponsored” killing.

The last time a Russian diplomat was killed was in 1985, at the peak of what became known as the Lebanese hostage crisis. Four Soviet diplomats and embassy workers were taken hostage by gunmen in September of that year, and one of them, Arkady Katkov, was later shot dead.

Some of the worst attacks on diplomats and officials have targeted Americans. In 1979, U.S. ambassador to Afghanistan Adolph Dubs was abducted by militants and later killed during a rescue attempt. Despite pleas not to attempt a violent rescue mission by the U.S., Afghan officials went ahead — a decision that was harshly criticized at the time by the Carter administration. Apart from Dubs, five other U.S. ambassadors have been killed in armed attacks while in office.

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The most recent — and probably most consequential — killing of a U.S. diplomat occurred in Sept. 2012, when U.S. ambassador to Libya Christopher Stevens was assassinated at the U.S. Special Mission in Benghazi. Three other Americans were also killed during the attack. Then-Secretary of State Hillary Clinton has faced heavy criticism for what some considered her personal failure to protect the ambassador with adequate security personnel.

Attacks on diplomatic personnel have struck not only larger nations such as the United States and Russia. Perhaps the most high-profile case was the downing of a plane carrying Rwandan President Juvénal Habyarimana and Burundian President Cyprien Ntaryamira in 1994.

There is disagreement on who destroyed the plane, which was shot down with a surface-to-air missile. A 2010 Rwandan government inquiry blamed Hutu extremists, who later killed 800,000 people from the Tutsi ethnic group along with moderate Hutus in one of the worst genocides in history. A French governmental inquiry, however, came to the opposite conclusion: It blamed the Rwandan Patriotic Front, the party of current Rwandan President Paul Kagame, which ended the genocide after 100 days.