Senate Finance Committee ranking member Ron Wyden Ronald (Ron) Lee WydenDemocrats call for declassifying election threats after briefing by Trump officials Read Democrats' report countering Republicans' Biden investigation Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (D-Ore.) sent a letter to the IRS commissioner Wednesday about a confidential memo that reportedly contradicts the Trump administration's reasons for not complying with a congressional subpoena for President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE's tax returns.

Wyden wrote a letter to IRS Commissioner Charles Rettig asking him a series of questions including about when he first became aware of the memo that was reported last month by The Washington Post and who else in the IRS knew about it.

The Oregon Democrat said in his letter to Rettig that the commissioner, in testimony before a House Appropriations subcommittee, "intimated that you had not discussed with IRS legal staff whether you as IRS Commissioner have discretion over whether to comply with a request for tax information."

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Wyden added that the reported existence of the memo, which was prepared in 2018, according to the Post, "may have significant implications on your House Appropriations subcommittee testimony" and his prior response to a letter by Wyden.

Wyden said in a statement that “Congress needs to investigate to know whether Trump appointees have engaged in a cover up to conceal political interference inside the IRS."

"Career IRS officials prepared a legal analysis stating that the agency was required to provide the returns, and it was buried. We need to know who saw this memo and when," he added.

The draft memo written by an IRS lawyer last fall determined that the agency has to provide tax returns that are requested by Congress unless the president invokes executive privilege.

Treasury Secretary Steven Mnuchin Steven Terner MnuchinHillicon Valley: DOJ proposes tech liability shield reform to Congress | Treasury sanctions individuals, groups tied to Russian malign influence activities | House Republican introduces bill to set standards for self-driving cars Treasury: Trump's payroll tax deferral won't hurt Social Security Treasury sanctions individuals, groups tied to Russian malign influence activities MORE last month rejected a subpoena for the president's tax returns. He has said the request lacks a legitimate legislative purpose.

President Trump has not invoked executive privilege over his tax returns, according to officials.

The Hill has reached out to the IRS for comment.