Denis Voronenkov, who testified to Ukrainian investigators and criticized Russian policies after his move to Kiev last fall, was shot dead by an unidentified gunman near the entrance of an upscale hotel in the Ukrainian capital.
During the attack, Voronenkov's bodyguard, who fired back, was wounded.
Both were hospitalized but Ukrainian officials said the gunman, who they claimed was a Ukrainian citizen, later died from wounds in his chest and head.
The killing prompted an angry exchange between the two countries whose relations have soured badly in recent years following Russia's annexation of the Crimea peninsula from Ukraine and the conflict in eastern Ukraine.
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Ukrainian President Petro Poroshenko said Voronenkov's killing was an "act of state terrorism" that "clearly shows the handwriting of Russian special services shown repeatedly in various European capitals in the past."
Russian President Vladimir Putin's spokesman, Dmitry Peskov, dismissed the claim as "absurd" in a statement carried by Russian news agencies.
Russian Foreign Ministry spokeswoman Maria Zakharova went further, saying the "killer regime" in Kiev "will do its best to make sure that no one will ever know the truth about what happened."
Several hours after Voronenkov was killed, a team of investigators and police were seen working at the front door of the Premier Palace hotel, which is frequented by Kiev's rich and powerful.
Ukraine's chief prosecutor, Yuriy Lutsenko, said Voronenkov was killed shortly before a meeting with another fugitive Russian lawmaker, Ilya Ponomaryov.
Both men were scheduled to give testimony later in the day at Ukraine's Military Prosecutor's Office. The purpose of the testimony was not immediately clear.
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