Boris Nemtsov, a leading critic of Russian President Vladimir Putin, was apparently murdered because he planned to reveal evidence of Russia’s role in the Ukraine conflict, Ukrainian President Petro Poroshenko said Saturday. Poroshenko said Nemtsov had told him weeks ago he had proof of Russia’s involvement and would disclose it, Reuters reported.

"He said he would reveal persuasive evidence of the involvement of Russian armed forces in Ukraine. Someone was very afraid of this,” Poroshenko reportedly said Saturday in televised remarks during a visit to the city of Vinnytsia.“They killed him.”

In a recent interview with the magazine Sobesednik, Nemtsov said he feared Putin would have him killed because of his opposition to the war in Ukraine, the New York Times reported. Nemtsov, 55, was killed in Moscow Friday night after unidentified gunmen shot the Russian opposition politician as he was walking near the Kremlin. He was a former Russian deputy prime minister and once considered a likely candidate for president.

Putin quickly condemned the murder and assumed personal control over the investigation, according to the Washington Post. “Putin noted that this cruel murder has every sign of being a contract killing, which has a solely provocative nature,” Kremlin spokesman Dmitry Peskov reportedly told Russian news agencies.

Pro-Russian separatists rebelled in east Ukraine last April following the overthrow of a Kremlin-backed president in Kiev and Russia’s annexation of the Crimea peninsula. More than 5,000 people have been killed since then, the United Nations human rights office said last month, expressing fear that the real figure may be “considerably higher.”

Western nations have long accused Putin of fueling the pro-Russian rebellion in eastern Ukraine by funding and arming the separatists. Russia has denied the allegations.