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.
Anna Politkovskaya, a vocal critic of Russia's war in Chechnya, was shot four times in front of her Moscow apartment in 2006. A Moscow court sentenced five men to prison in 2014 for her death.
Human rights activist Natalya Estemirova was kidnapped outside her Chechnya home in July 2009 and was found dead later the same day. The head of the group Estemirova worked for, Memorial, accused the Kremlin-backed Chechen leadership of ordering her killing.
Russian human rights lawyer Stanislav Markelov, left, represented the family of a Chechen woman killed by a former Russian colonel in March 2000. He was shot in 2009. The gunman also killed Anastasia Baburova, right, who may have been trying to intervene.