Kremlin critic Denis Voronenkov was killed by a Russian agent, according to a senior Ukrainian official.
Mr Voronenkov, a former Russian legislator who became a vociferous critic of Moscow after his recent move to Ukraine, was shot dead near the entrance to an upscale hotel in the centre of Kiev on Thursday.
Anton Gerashchenko, an adviser to Ukraine's interior minister, identified the man who shot Mr Voronenkov as 28-year-old Pavel Parshov, and said he had been trained in Russia by Moscow's security services.
"He underwent a special course at a school for saboteurs," Mr Gerashchenko said.
Ukrainian President Petro Poroshenko said the murder was an "act of state terrorism", a charge denied by Russian officials.