Denis Voronenkov, a Kremlin-critic and former Russian MP who obtained Ukrainian citizenship in 2016, was shot dead in a presumed contract killing in central Kyiv on March 23. Ukraine's President Petro Poroshenko said the murder was an "act of state terrorism" committed by Russia.
Voronenkov and his bodyguard were attacked as they left the capital's Premier Palace hotel, news reports said. The bodyguard fired back at the assailant, who later died in hospital of gunshot wounds to the head and chest.
The attack bore the signs of a contract killing, Kyiv police chief Andriy Kryshchenko told 112 Ukraina TV, the BBC reported.
Voronenkov was a witnesses in the treason case against former Ukrainian president Viktor Yanukovych, indicted by authorities in Kyiv for treason by aiding Russian aggression in 2014. The deposed leader is accused of allowing Russian forces to take control of parts of Ukraine as his regime crumbled.
The former deputy in Russia's State Duma lower chamber of parliament was also a witness to contraband activities within Russia's Federal Security Service (FSB), according to the Ukrainian authorities.
The killing drew swift condemnation from the Ukrainian leadership, with Poroshenko calling the "insidious murder" in Kyiv "an act of state terrorism on the part of Russia", the country Voronenkov was forced to flee for political reasons.
"There is a clear pattern of the Russian secret services, which has been demonstrated repeatedly in various European capitals," presidential press secretary Sviatoslav Tseholko quoted Poroshenko as saying.
The president also linked the assassination to the series of explosions that swept through a military munitions depot near Ukraine's north-eastern city of Kharkiv since March 22, describing it as the "signature style of Russian special services"
The Kremlin denied any involvement Voronenkov's murder. "We believe that all the falsehoods that can already be heard about much-hyped Russian involvement are absurd," Reuters quoted Russian President Vladimir Putin's spokesman Dmitry Peskov as saying.
However, according to authorities in Kyiv, a main motive behind the killing could be to remove a witness to the Yanukovych case, as well as in the FSB contraband case, which was "covered up by the president of Russia", Ukrainian Prosecutor General Yuriy Lutsenko told a news briefing in Kyiv.
In a separate statement, Lutsenko said that Voronenkov's bodyguard was shot in the chest, but was alive and cooperating with investigators.
Meanwhile, another former Russian State Duma deputy Ilya Ponomaryov, another Kremlin critic currently living in Kyiv, says Voronenkov was killed on his way to a meeting with him. "He was on his way to a meeting with me. I am lost for words," Ponomaryov wrote on his Facebook page.
Voronenkov had been placed on a Russian federal wanted-list in connection with an alleged $5mn property fraud shortly before he fled from Russia.
According to Lutsenko, Ponomaryov and Voronenko's widow Maria Maksakova were to be placed under the protection of the SBU security service.