Questions have arisen about Manafort's previous work for the political party of the Kremlin-backed former Ukrainian leader, Viktor Yanukovich.
Federal investigators from the US Department of Justice are examining American ties to corruption in Ukraine, CNN reported on Friday. The investigation is looking at the work of Manafort's firm and another lobbying firm, the Podesta Group, headed by Tony Podesta, whose brother, John Podesta, is chairman of Clinton's presidential campaign.