The senator from the USA state of Iowa added that if necessary, he will subpoena Manafort.
But those close to Manafort maintain he's done nothing wrong - in fact, they think Manafort's involvement in this whole thing is completely unlike that of Trump Jr. and Jared Kushner, who was also at the meeting.
Trump Jr. found himself engulfed in scandal after releasing emails this week in which he accepted a meeting with a Russian attorney whom publicist Rob Goldstone, an acquaintance of Trump Jr., said had information representing efforts from the Russian government to help elect the elder Trump.
Trump Jr. chose a friendly venue, Fox News, to downplay the meeting Tuesday night. The Russian lawyer has claimed that the 30-minute meeting focused on the Magnitsky Act and the issue of Russian adoptions by U.S. citizens.
As reported in May, Special Counsel Bob Mueller is putting Manafort and former White House National Security Advisor Michael Flynn under a microscope as part of his investigation into Russian meddling in the 2016 presidential election. He had been accused of previously accepting millions of dollars in off-the-books payments from deposed Ukrainian President Victor Yanukovych's Party of Regions while working as an adviser to the former head of state.
In an interview before departing Wednesday evening for France, Trump told Reuters that he didn't know about the meeting "until a couple of days ago when I heard about this". Russian Federation denies meddling in the campaign, and Trump says there was no collusion.
But Trump also refused to dispute an assertion made by Russian President Vladimir Putin that Trump believed him when he said that Russia hadn't meddle in the election during their lengthy meeting in Germany last week. Trump Jr. said the meeting "really went nowhere" and that he never told his father about it because there was "nothing to tell".
"The information they suggested they had about Hillary Clinton I thought was Political Opposition Research", Trump Jr. wrote in a statement that was published alongside the correspondence.
Investigations into possible collusion between Russian Federation and the Trump campaign have shadowed the White House for months.