Kommersant via Getty Images

Russian mourners have laid flowers near the site where Boris Nemtsov, one of Russia's most prominent opposition leaders, was gunned down Friday night.

Nemtsov was shot four times in the back while walking across the Bolshoy Moskvoretsky Bridge, near the Kremlin. A former Deputy Prime Minister under President Boris Yeltsin, he had become a vocal opponent of President Vladimir Putin and the war in Ukraine. His last tweet was an appeal for Russians to join in an anti-war march planned for Sunday in Moscow.

In a recent interview, Nemtsov had expressed fears that Putin would have him killed because of his protests to the war in Ukraine. His death marks the highest profile assassination during Putin's tenure.

Russian officials have suggested a few possible theories for the murder, including that members of the opposition party may have planned it as "a provocation aimed at destabilizing the country," according to the BBC. The investigation committee also cited Islamic extremism as a possible motivation for the crime.

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A statement from the Kremlin noted that the "cruel murder has all the signs of a contract killing and carries an exclusively provocative character," while Putin's spokesman said the president would personally lead the investigation. President Obama, meanwhile, honored Nemtsov as a "tireless advocate" and called on the Russian government to "ensure that those responsible for this vicious killing are brought to justice."

Supporters grieving Nemtsov, who began his career as an atomic physicist, left tributes at the site of the shooting throughout the night. The images above have been geo-tagged near St. Basil's Cathedral, steps from the Kremlin in Moscow.

@fusion