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.

___

ANNA POLITKOVSKAYA

Renowned journalist Anna Politkovskaya, 48, was fatally shot in the elevator of her Moscow apartment building in October 2006. Her work in the Novaya Gazeta newspaper was sharply critical of Kremlin policies in Chechnya and of human rights violations there.

Last year, a court convicted five men, most of them Chechens, of involvement in the murder. However, Russia's Investigative Committee has said it is still trying to determine who ordered the killing.

___

ALEXANDER LITVINENKO

Former Russian intelligence officer Alexander Litvinenko, 44, became sick after drinking tea laced with radioactive polonium-210 at a London hotel in November 2006 and died three weeks later. Litvinenko had fallen out with the Russian government and became a strong critic of the Kremlin, obtaining political asylum after coming to Britain in 2000.

Two weeks before he was poisoned, Litvinenko blamed Putin for the murder of Politkovskaya. Before he died, he signed a statement blaming Putin for his poisoning.

British police have named two Russian men, former KGB agent Andrei Lugovoi and Dmitry Kovtun, as prime suspects. They deny involvement, and Russia refused to extradite them. An inquiry in Britain is now examining the circumstances of Litvinenko's death.

___

STANISLAV MARKELOV

Stanislav Markelov, a human rights lawyer, was shot after leaving a news conference less than half a mile from the Kremlin in January 2009. Markelov, 34, was appealing the early release of Yuri Budanov, a Russian military officer convicted of killing a young Chechen woman. A journalist walking with Markelov, Anastasia Baburova, also died in the attack. A Russian nationalist extremist was sentenced to life in prison for the killings.

___

NATALYA ESTEMIROVA

Human rights activist Natalya Estemirova, 50, was abducted in Chechnya in July 2009 and found shot dead the same day. One of Chechnya's best known rights activists, Estemirova headed the Memorial group's Chechen branch and exposed alleged abuses by the forces of Kremlin-backed Chechen President Ramzan Kadyrov.

Russian investigators said in 2010 that two brothers who were members of an Islamic militant group killed Estemirova, who had implicated them in kidnappings of Chechen civilians. Memorial said DNA evidence showed that the two men — one of whom was killed in 2009 and the other granted asylum in France — didn't commit the crime.

___

BORIS NEMTSOV

Boris Nemtsov, 55, who served as a deputy prime minister in the 1990s and became a prominent opposition figure under Putin, was gunned down in Moscow on Friday night. The killing came a few hours after he denounced Putin's "mad, aggressive" policies and the day before he was to help lead a rally protesting Russia's actions in the Ukraine crisis and the economic crisis at home.

Russia's top investigative body said it is looking into several possible motives including an attempt to destabilize the state, Islamic extremism, the conflict in Ukraine and his personal life.