Denis Voronenkov
Former lawmaker of the Russian State Duma Denis Voronenkov and his wife Maria Maksakova pose for a picture during a screening of the movie "Nemtsov" by Vladimir Kara-Murza in Kiev, Ukraine Feb. 27, 2017. Reuters

To add to the number of people killed for criticizing Russian President Vladimir Putin and government, another Kremlin critic and former Russian lawmaker's death Thursday, has left everyone shocked. Ukrainian President Petro Poroshenko called the killing a "Russian state terrorist act", according to reports.

Denis Voronenkov, who had been a Communist member of Russia's lower legislative house before he stepped down, was shot outside a hotel in Kiev during daytime, officials said. The suspect in Voronenkov's death, died in the hospital after a shootout with Voronenkov's bodyguard. Voronenkov and his wife, former Russian lawmaker Maria Maksakova, were critics of Russia's seizure of Crimea, CNN reported.

Read: Did Putin Critic, Vladimir Kara-Murza, Get Poisoned?

Voronenkov is one of several Kremlin critics who was injured or killed under mysterious circumstances. In February 2015, Putin critic Boris Nemtsov, a deputy prime minister in the late 1990s under President Boris Yeltsin, was shot at in central Moscow. He was killed hours after appealing for support for a march in Moscow against the war in Ukraine. In one of his interviews before he died, he expressed his fear of Putin having him killed because of his opposition to the war in Ukraine, according to BBC.

Last month, Nemtsov's friend, Vladimir Kara-Murza, leader of the Russian opposition, who had been a vocal critic of Kremlin's policy of assassinating political enemies, went into a state of coma caused by an unknown poison, as reported by the New York Times.

In 2013, Russian oligarch Boris Berezovsky was found dead inside his house in Britain with a noose around his neck. Acquaintances had indicated that he had been depressed after losing billions in legal action against his former business partner, after which he was living in exile in Britain since 2000, the Telegraph reported. However, Berezovsky's daughter had suggested that the Russian government could have been behind his death, according to the Guardian.

In July 2009, 51-year-old human rights activist Natalya Estemirova was found dead after being kidnapped outside her home in the Russian republic of Chechnya. She had collected evidence of human rights abuses in Chechnya since the start of the second war there in 1999, according to Sky News.

In November 2006, former intelligence officer-turned-Putin critic Alexander Litvinenko, 43, died after allegedly drinking tea laced with radioactive isotope polonium in a hotel. He was living in London in exile, according to RTE News.