The House Ways and Means Committee said it had received “credible allegations” from a federal employee of potentially “inappropriate efforts to influence” the IRS’ mandatory audit of presidential tax returns.

References to the unexplained allegations were in a letter included in a Tuesday filing by the committee in its federal lawsuit against Treasury Secretary Steven Mnuchin and IRS Commissioner Charles Rettig. The filing consisted of arguments in support of the committee’s motion for the court to grant it summary judgment in its lawsuit seeking six years of tax returns from President Trump and from eight of his businesses.

In an Aug. 8 letter to Mnuchin, Ways and Means Chairman Richard E. Neal said the committee had received an “unsolicited communication” July 29 from a federal employee claiming “evidence of possible misconduct” regarding the presidential audit program. While not required to do so by law, the IRS adopted the practice during President Richard Nixon’s term of annually auditing the president’s tax returns.

Neal’s letter provides no details of the allegations and doesn’t specify that the allegations regarding the audit program related to Trump’s returns.

But in his letter, Neal, a Massachusetts Democrat, characterized the allegations as “a grave charge that appreciably heightens the Committee’s concerns about the absence of appropriate safeguards as part of the mandatory audit program and whether statutory codification of such program or other remedial, legislative measures are warranted.”