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Firtash, 48, is one of Ukraine's richest men, an oligarch whose close links to Russia and involvement in the gas, chemicals, media and banking sectors gave him substantial influence, notably during the administration of recently ousted, Moscow-backed President Viktor Yanukovich.
The Federal Criminal Office, had identified the man taken into custody only as Dmitry F. and said he had been under investigation by the U.S. Federal Bureau of Investigation since 2006.
(Reporting by Michael Shields and Angelika Gruber; Editing by Louise Ireland)