LONDON -- The Security Service of Ukraine -- the main tasks of which include counter-intelligence operations -- is involved in the investigation into the Saturday killing of the former speaker of the Ukrainian parliament, President Volodymyr Zelenskyy said.

       Andriy Parubiy, who chaired the Rada -- Ukraine's unicameral parliament -- from April 2016 to August 2019, was shot dead by an unidentified assailant in the western city of Lviv on Saturday.

       In posts to social media, Zelenskyy called Parubiy's death "a horrific murder" and said the search for the perpetrator was ongoing.

       "Many resources are deployed -- all that's necessary," the president said in a Saturday post to Telegram. "The crime was, unfortunately, carefully prepared. But every effort is being made to solve this crime."

       Zelenskyy said he spoke with the head of the SBU, Vasyl Maliuk, about the incident. "The Security Service of Ukraine is engaged in the investigation. I instructed that verified information be promptly communicated to the public," he wrote.

       Parubiy was a prominent pro-European Union and pro-NATO politician who was among the leaders of the 2014 Maidan Revolution, which saw a grassroots pro-Western protest movement topple pro-Russian President Viktor Yanukovych.

       After the revolution, Parubiy served as the secretary of Ukraine's National Security and Defence Council in 2014, during which time Russian forces seized Crimea and sparked conflict in the eastern Donbas region.

       ABC News' Oleskiy Pshemyskiy and Morgan Winsor contributed to this report.