Russian opposition leader Boris Nemtsov was killed in Moscow Friday night after unidentified attackers shot the politician as he was walking near the Kremlin. The former Russian deputy prime minister was once considered a likely candidate for president and was known for being a leading critic of Russian President Vladimir Putin, who he said he feared would have him killed because of his opposition to the war in Ukraine, according to remarks he made in a recent interview, the BBC reported.

Nemtsov, 55, first rose to prominence as the governor of the city of Nizhny Novgorod and his tenure there gained him a reputation as an economic reformer, the BBC said. The politician subsequently became deputy prime minister under the government of former Russian President Boris Yeltsin in the late 1990s but fell out of favor with Putin, who succeeded Yeltsin in 2000. Nemtsov founded a number of opposition parties after his departure from parliament in 2003, most recently including the Republican Party of Russia – People’s Freedom Party.

At least seven shots were fired at Nemtsov by assailants in a passing car, said a spokesman for the Russian Investigative Committee, according to the Washington Post. Investigators are currently on the scene and questioning witnesses of the shooting death. Dmitry Peskov, a spokesman for Putin, said the leader had assumed personal control of the murder investigation, adding that, “Putin noted that this cruel murder has every sign of being a contract killing, which has a solely provocative nature.”

Nemtsov had been due to lead a mass rally against Putin and the war in Ukraine this Sunday. The rally’s organizers had issued a statement earlier condemning Putin’s role in Russia’s Ukraine involvement as well as the country’s expanding economic crisis, the New York Times reported. Nemtsov’s criticisms of Putin, however, predate the current conflict in Ukraine, which many of Putin’s critics accuse the leader of engineering. Nemtsov previously criticized Putin for corruption, notably in recent years leading up to the 2014 Sochi Winter Olympics. In his last tweet, Nemtsov called for Russia’s opposition to unite against Putin.