Alexei Navalny is the latest Putin critic to die in suspicious circumstances

Protesters light candles on Friday in front of the Russian Embassy in Prague after the announcement that the Kremlin's most prominent critic, Alexei Navalny, had died in an Arctic prison. (Milan Kammermayer/AFP via Getty Images)

The death of Russian opposition leader Alexei Navalny in a penal colony in Russia's Arctic north was shocking, but hardly surprising.

For years, critics and opponents of Russian leader Vladimir Putin have fallen victim to shootings, poisoning with radioactive or nerve agents, or have plunged to their deaths from open windows.