The inspector general for the Treasury Department told lawmakers this week Treasury Secretary Steven Mnuchin properly withheld President Donald Trump's federal tax returns as he relied on an opinion from the Justice Department's Office of Legal Counsel, raising questions about whether that same legal opinion will hold up in a federal court case over the tax documents.

In a cover letter sent this week to the House Ways and Means Committee and released Friday by Senate Finance Committee Chairman Chuck Grassley, acting Inspector General Richard Delmar wrote the inquiry found the Treasury Department properly processed the request for the returns, and after seeking guidance from OLC, "determined that it was bound by that office's advice, and based on that advice determined not to provide the returns and tax information sought."

"We do not presume to opine on the analysis and conclusions of the OLC opinion and advice," Delmar wrote. "We report the Treasury decision to conform to that advice, and do not have a basis to question that decision."

The OLC opinion Mnuchin relied on found lawmakers lacked a "legitimate legislative purpose" in subpoenaing the president's federal tax returns, and therefore he could not hand over the documents.

In a five-page memo laying out the inquiry, Sally Luttrell, the assistant inspector general for investigations, wrote the investigation "did not examine the legal opinions in formulating responses to Chairman [Richard] Neal, nor attempt to identify or interpret the basis for Treasury's decisions to produce, or not produce, records." She said that one official interviewed in the process said that OLC "was consulted due to the legality of Chairman Neal's request," referring to the top Democrat on the House Ways and Means Committee.

"It is our understanding that the underlying issue regarding production is currently in litigation," Luttrell added.

The IG report comes as the Democratic-controlled House Ways and Means Committee pursues a federal lawsuit seeking Trump's tax returns.

U.S. District Judge Trevor McFadden of the District of Columbia has stayed the case pending a resolution in another House lawsuit for former White House counsel Don McGahn's testimony, as a panel opinion issued by the U.S. Court of Appeals for the D.C. Circuit in late February found the House could not seek judicial enforcement of a subpoena.

The D.C. Circuit soon after granted an en banc rehearing of the case—set to be held later this month—and vacated that original panel opinion.

Justice Department lawyers are sure to point to the IG report in arguing the administration does not have to release the president's federal tax returns.

However, other federal judges have questioned OLC opinions, noting they do not hold the force of law. U.S. District Chief District Judge Beryl Howell of the District of Columbia raised that point last year during oral arguments in a House lawsuit seeking grand jury materials redacted from special counsel Robert Mueller III's report, as did U.S. District Judge Victor Marrero as he dismissed a lawsuit from Trump seeking to block enforcement of a subpoena for the president's tax documents.

"The court is not persuaded that it should accord the weight and legal force the president ascribes to the DOJ memos, or accept as controlling the far-reaching proposition for which they are cited in the context of the controversy at hand," Marrero wrote in the October opinion. That case is now before the U.S. Supreme Court.

Exactly how McFadden, a Trump appointee who joined the bench in 2017, will address the OLC opinion in this case is an open question.

In the memo released Friday, Luttrell wrote an official in the Treasury Department's Office of General Counsel "stated Secretary Mnuchin, informed by legal advice from OGC, made the final decision to rely on DOJ OLC's opinion."

"The same official said the DOJ OLC opinion is 'binding' because, as described, DOJ OLC is the counsel for the executive branch," the memo reads. "A second OGC official specified it was a matter of 'complying with the law' in respect to Treasury's actions to rely [on] DOJ OLC's opinion."

Luttrell's memo also said the Treasury's general counsel received an "unsolicited letter" from Trump's private attorney, William Consovoy of Consovoy McCarthy, "regarding Chairman Neal's requests for the president's tax information."

"OGC leadership stated the letter did not affect Treasury or OGC in any manner," Luttrell wrote. "In addition, all of the OGC officials we asked about, or who were aware of the letter, stated it had no effect on Treasury or OGC's processing, decisions, or responses. Treasury produced a copy of this letter from Consovoy in response to one of Chairman Neal's requests for records."

The memo said there was no indication "of any other unsolicited opinions or attempts to influence the process."

Democrats have criticized the Trump administration for its use of OLC opinions, particularly over the transmission of the whistleblower report on Trump's call with Ukrainian President Volodymyr Zelensky.

The Trump administration later released a memo of Trump's call with Zelensky, and House Democrats released a redacted version of the whistleblower report of the call. Those Ukraine-tied allegations built the basis of Democrats' impeachment inquiry last year. The Senate acquitted Trump in early February.

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