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 Msocow'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.
Mr Voronenkov was wanted on criminal charges in Russia.
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, told reporters Mr Gerashchenko's allegation was "absurd".
Ukraine's chief prosecutor, Yuriy Lutsenko, said Mr Voronenkov was killed shortly before meeting another fugitive Russian legislator, Ilya Ponomaryov.
Both men had been scheduled to give evidence at Ukraine's Military Prosecutor's Office. The purpose of the hearing was not immediately clear.
Ukrainian media on Friday published leaked CCTV footage of the attack. It shows the killer shooting Mr Voronenkov from behind as he walks down the street with a bodyguard.
When the bodyguard tries to intervene, he is also shot, leaving the killer free to shoot Mr Voronenkov again as he lies on the floor.
The injured bodyguard then pulls out his gun and, while lying on the floor, opens fire on the killer, who died later in hospital.