Grassley noted he intends to question Manafort on the Foreign Agent Registration Act, including on his role in the meeting between Donald Trump Jr. and Russian lawyer Natalia Veselnitskaya in June 2016.
Trump Jr. on Tuesday morning posted a series of emails between himself and Rob Goldstone, a music publicist who contacted Trump Jr. and told him about an opportunity to receive potentially damaging information about Clinton as part of a Russian government effort to help his father's campaign.
Manafort resigned from Trump's campaign in August, long before the presidential election in November.
The New York Times has reported during the past several days the meeting was arranged on the premise of passing damaging information about Democratic nominee Hillary Clinton from the Russian government to top members of the Trump campaign.
Trump Jr. told Fox News' Sean Hannity Tuesday night that he would be willing to testify under oath about the meeting, which he said yielded no useful information and instead focused on adoptions.
His panel is investigating Russian meddling in the US election.
Although he's entangled in the larger web created by the Russian Federation probes, Manafort previously disclosed this meeting to congressional investigators before Trump Jr. leaked his emails - but he wasn't required to disclose the content of it.
Grassley lead the hearing as committee chairman, starting off with an opening statement that detailed job duties of the Federal Bureau of Investigation director and Wray's credentials.
The Iowa Republican told reporters he thinks there has been "lackadaisical enforcement" by the Trump and Obama administrations. Diane Feinstein of California are working with Special Counsel Robert Mueller to ensure Manafort's appearance does not conflict with any criminal investigation being overseen by Special Counsel Robert Mueller.
The president's attorney, Jay Sekulow, said in an interview with NBC's "Today" that Trump Jr. did not violate any laws by accepting the meeting.
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. He was told the Russian government had information that could "incriminate" Clinton and her dealings with Russia. Trump Jr. responds: "I love it".