Russia's opposition politician Boris Nemtsov is the latest in a list of Kremlin opponents who have been killed or died in suspicious circumstances in the past decade.
Here's a list of other high-profile cases:
1. Liberal Russia party co-chairman Sergei Yushenkov
The 52-year-old politician was gunned down at the entrance of his apartment in Moscow in 2003. He was the ninth member of parliament to be shot dead since 1994, out of 53 assassination attempts recorded by the interior ministry at that time. Yushenkov had probed a spate of devastating apartment bombings in 1999 which provided the pretext for the second Chechen War that helped propel Russian President Vladimir Putin to power.
2. American investigative journalist Paul Klebnikov
He was fatally shot on a street in Moscow in 2004. The head of Forbes magazine's Russian edition had reportedly angered Russian oligarch Boris Berezovsky with his reporting. In an article "Godfather of the Kremlin", he accused Berezovsky of operating like a mafia boss. Klebnikov was also known for his investigative reporting on the war in Chechnya. It remains a mystery who killed him.
3. Russian oligarch Boris Berezovsky
A fierce foe of Putin who co-founded Liberal Russia with Yushenkov, Berezovsky, 67 was found hanged in 2013 in a bathroom at his country home in the UK, where he was living in self-imposed exile. Coroners were unable to confirm his death was suicide despite strong evidence that he had killed himself.
4. Financial lawyer Sergei Magnitsky
Magnitsky, who denounced official corruption, died in 2009 after being denied medical treatment in a Moscow jail. He was 37. His death sparked a crisis in US-Russia relations after Washington passed the so-called "Magnitsky Act" to target Russian officials implicated in his demise.
5. Former intelligence officer Alexander Litvinenko
Litvinenko, 43, died in 2006 after allegedly drinking tea laced with radioactive isotope polonium in a hotel restaurant in London, where he was living in exile. Two Russian businessmen and ex-KGB agents Dmitry Kovtun and Andrei Lugovoi were accused of poisoning him by British authorities but Russia refused to extradite them. Litvinenko, an associate of Berezovsky, had supposedly accused Putin of being corrupt.
6. Russian journalist Anna Politkovskaya
The 48-year-old award-winning journalist was shot dead in 2006 at the entrance to her apartment in central Moscow on Putin's 54th birthday. She had written books including "Putin's Russia: Life in a Failing Democracy" and also investigated human rights abuses in Chechnya. Five men were found guilty of carrying out the hit but it was not known who ordered the murder.
7. Human rights lawyer Stanislav Markelov and opposition journalist Anastasia Baburova
The prominent lawyer was gunned down in the middle of a Moscow street after leaving a press conference in 2009. Opposition journalist Anastasia Baburova, 25, who was with him, was also shot dead. Markelov, 34, who had represented victims of a 2002 Moscow theatre siege whereby over 100 hostages were killed by Russian special forces, was assumed to be the main target. He also worked for opposition newspaper Novaya Gazeta and had previously dealt with the murder case of the paper's correspondent Politkovskaya.
8. Human rights activist Natalia Estemirova
The 50-year-old activist was kidnapped in 2009 while investigating government abuses in the volatile North Caucasus region of Chechnya. She was found dead several hours later.
Sources: AFP, Bloomberg, BBC, The Telegraph, The Guardian