Sen. Amy Klobuchar (D-Minn.) on Thursday asked special counsel Robert Mueller whether his team reviewed President Trump's taxes as part of their investigation into Russian election interference, something Trump has said he assumes Muller's team studied.

Klobuchar, who is running for the 2020 Democratic presidential nomination, wrote to Mueller seeking information on whether investigators obtained Trump's personal tax returns or financial statements from the Trump Organization.

The senator, who is a member of the Senate Judiciary Committee, further asked that Mueller turn over any copies of Trump's taxes or financial statements to the Senate panel as part of its review of the special counsel's full report.

Klobuchar sent her letter to Mueller one day after she raised the issue of Trump's taxes during a contentious hearing with Attorney General William Barr.

Barr told the Minnesota Democrat that he did not know whether Mueller had reviewed Trump's taxes, saying Klobuchar could submit a written question to be answered at a later time "or you could ask Bob Mueller when he comes here.”

Klobuchar indicated that she planned to ask Muller about the issue.

Senate Judiciary Committee Chairman Lindsey Graham (R-S.C.) has said he does not plan to call Mueller to testify before the Senate panel.

Trump last week said he assumed that Mueller looked at his finances as part of the investigation into Russian interference in the 2016 election.

"They checked my financials, and they checked my taxes, I assume," he said. "It was the most thorough investigation probably in the history of our country."

The president's comment came as he argued his cooperation with the Russia investigation was sufficient and that he did not need to comply with Democratic subpoenas.

Still, Mueller's 448-page report, a redacted version of which was released last month, made no mention of Trump's taxes or his business records.

Democrats on the House Ways and Means Committee have requested the Treasury Department turn over six years' worth of Trump’s personal and business tax returns. The department has missed two deadlines to do so thus far.

Treasury Secretary Steven Mnuchin said recently he expects Treasury to provide the House Ways and Means Committee with a final decision by May 6 on whether the department will comply with the request after receiving legal conclusions from the Department of Justice.

Trump has insisted he will not voluntarily give up his returns. He has cited an ongoing audit, the same reason he gave when he broke with years of precedent and declined to release his returns during the 2016 presidential campaign.