People light candles in memory of Boris Nemtsov, seen behind, at the monument of political prisoners 'Solovetsky Stone' in central St.Petersburg, Russia, Saturday, Feb. 28, 2015. Nemtsov was gunned down Saturday near the Kremlin, just a day before a planned protest against the government. (AP Photo/Dmitry Lovetsky) (The Associated Press)
MOSCOW – Prominent Russian opposition figure Boris Nemtsov's killing follows the slaying over the past decade of several other high-profile critics of President Vladimir Putin and his policies. Here is a look at some of the best-known cases.