Report: Boris Nemtsov, outspoken critic of Putin, shot dead in Moscow

Russian opposition leader Boris Nemtsov was shot four times and killed by an unknown assailant in central Moscow on Friday evening, a law enforcement agency told Russian state news agency Itar Tass.

He was deputy prime minister in the late 1990s under President Boris Yeltsin and had been one of current President Vladimir Putin's most vocal critics.

One of Nemtsov's criticisms was over the 2014 Winter Olympics held in Sochi. Nemtsov published a report in 2013 describing the Sochi games as one of the most "outrageous swindles" in recent Russian history. He claimed that up to 60% of the final cost -- or $30 billion -- had been embezzled.