Sen. Richard Blumenthal (D-Conn.) said that Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE Jr.'s interview with a Senate panel Thursday had "gaping holes" and the president's son should return to testify under oath on his involvement with Russia during the 2016 presidential campaign.

Donald Trump Jr. "left more questions unanswered than he answered," Blumenthal told MSNBC's Chris Matthews about his Senate Judiciary Committee testimony.

"There were gaping holes in the information he provided, which is why we need to have him come back in public under oath in a hearing of the Judiciary Committee."

The president's son answered questions by the committee for five hours on Thursday on his knowledge of Russian involvement in the election and a June meeting with a Russian attorney where he hoped to gather damaging information against then-Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE.

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"But he left nothing to dispel my impression that that June 9 meeting essentially signaled to the Russians 'we're open for business, we're ready to deal,' and he went to that meeting because he was promised, in effect, dirt on Hillary Clinton," Blumenthal said.

The senator, a former federal prosecutor, said he would gladly take the opportunity to question Trump Jr. in a public session.

"The president also has to be held accountable, because he is the one who is embroiled and involved in this issue, and the question is, 'What did he know and when did he know it?' " Blumenthal said.

Sen. Kamala Harris Kamala HarrisHarris honors Ginsburg, visits Supreme Court The Hill's Campaign Report: Trump and Biden vie for Minnesota | Early voting begins in four states | Blue state GOP governors back Susan Collins Kamala Harris: Black Americans have been 'disproportionately harmed' by Trump MORE (D-Calif.) also said that a testimony by the president was not off the table in the ongoing Russia investigation.