Former Russian agent Alexander Litvinenko was poisoned by a lethal dose of radioactive polonium, his tea spiked during a meeting with two former Russian security servicemen. He had said the Russian Federal Security Service orchestrated a series of bombings in Russia in 1999 that left hundreds dead and led to Russia's invasion of Chechnya later that year.
Russian opposition leader Boris Nemtsov was gunned down near the Kremlin on February 27, 2015. The killing took place two days before he was supposed to lead an opposition rally.
Business magnate Mikhail Khodorkovsky supported an opposition party and accused Putin of corruption. He spent more than 10 years behind bars, accused of tax evasion and fraud.
Anti-corruption lawyer Alexey Navalny once branded Putin's United Russia party "the party of crooks and thieves." He was arrested in December 2014 on accusations of fraud.
Russian businessman Boris Berezovsky accused the Kremlin of killing Alexander Litvinenko in 2006. In 2013, he was found dead with a noose around his neck; the coroner could not say whether it was a suicide.