Two senators are looking into a whistleblower’s allegations that at least one political appointee at the Treasury Department may have tried to interfere with an audit of President Trump or Vice President Pence, according to two people with knowledge of the matter, a sign that lawmakers are moving to investigate the complaint lodged by a senior staffer at the Internal Revenue Service.

Staff members for Sens. Charles E. Grassley (R-Iowa) and Ron Wyden (Ore.), the chairman and ranking Democrat on the Senate Finance Committee, met with the IRS whistleblower earlier this month, those people said. Follow-up interviews are expected to further explore the whistleblower’s allegations.

It could not be learned to what extent the senators consider the whistleblower a credible source. Trump administration officials have previously played down the complaint’s significance and suggested that it is politically motivated.

The whistleblower, a career IRS official, initially filed a complaint in July, reporting that he was told that at least one Treasury political appointee attempted to improperly interfere with the annual audit of the president’s or vice president’s tax returns. In recent weeks, the whistleblower filed additional documentation related to the original complaint, which was given to congressional officials in July, the two people said. These people spoke on the condition of anonymity because of the sensitivity of the complaint, which pertains to a confidential IRS audit that cannot be disclosed under federal law.

The contents of the additional information provided by the whistleblower were not known.

The IRS whistleblower complaint was first disclosed in an August court filing by Rep. Richard E. Neal (D-Mass.), the chairman of the House Ways and Means Committee. It raises the prospect that Trump administration officials at Treasury tried to improperly interfere with the IRS audit process. That process is supposed to be walled off from political interference.

Neal made the disclosure in court filings as part of his battle with the Trump administration over the president’s tax returns, which the Treasury Department has refused to furnish. At the time, Neal said the whistleblower complaint raises “serious and urgent concerns” about the integrity of the IRS audit process. A person close to Neal, who spoke on the condition of anonymity, said that the office’s position on the complaint is unchanged and that its investigation is ongoing.

The Treasury inspector general has opened a review of the Treasury Department’s handling of House Democrats’ request for Trump’s tax returns. Asked whether that review would look at the IRS whistleblower’s complaint, Rich Delmar, the acting inspector general, said in an email that “the inquiry is ongoing, and will take into account that aspects of the underlying matter are the subject of litigation.” The whistleblower also previously told The Washington Post that he had sent his complaint to Grassley, Neal and the Treasury Department inspector general for tax administration, a separate watchdog at Treasury.

Spokespeople for Grassley and Wyden declined to confirm their meeting with the whistleblower, citing strict federal privacy laws related to taxpayer information. The White House, the Treasury Department and the Treasury Department inspector general for tax administration also declined to comment.

In a statement in October, Grassley appeared to criticize Neal for not doing enough to verify the complaint before disclosing details about it, saying in a statement, “Talking about the existence of a complaint before taking the time to speak with the whistleblower or follow up on the whistleblower’s complaint is irresponsible.”

Wyden also demanded a Senate investigation of the whistleblower’s complaint in early October, writing on Twitter, “A bipartisan committee effort to get to the bottom of this should have been started months ago.”

Congressional investigations of the IRS whistleblower came as a separate whistleblower surfaced claims that Trump withheld military funding and other support for the newly elected president of Ukraine in exchange for an announcement that Ukraine was investigating former vice president Joe Biden and his son Hunter Biden. Those claims have set off a House impeachment inquiry that began public hearings last week.

The IRS whistleblower deals with a more technical but still potentially controversial topic: the handling of the annual tax audits of the president and vice president. The initial IRS complaint, filed by a career IRS official, relays concerns from another IRS official that at least one Treasury Department political appointee tried to improperly interfere with the annual tax audit of Trump or Pence.

There has been extreme focus on Trump’s tax returns and less scrutiny of Pence, but the presidential audit program applies to both offices. It could not be learned which audit was the subject of the complaint. It is a violation of federal law to reveal details about the status of taxpayer information.

“Obviously, it’s serious anytime an allegation of this nature is leveled. That said, it has to be substantiated and there has to be an understanding of what contact took place,” said Mark Everson, who served as the IRS commissioner during the George W. Bush administration.

Trump has faced demands to release his tax returns since the 2016 presidential election, when he broke with decades of precedent by refusing to release them. He has claimed that he is under audit and that it would be improper to release the returns while that is the case.

On Thursday, Trump’s private attorneys asked the Supreme Court to intervene to block a New York prosecutor’s attempts to obtain the president’s tax returns.

Trump also faces an ongoing lawsuit from the House Ways and Means Committee, which has demanded access to his returns, as well as investigations by other House panels.

Democrats have argued that the Treasury Department is obligated to turn over Trump’s tax returns under a 1924 law that explicitly gives the House and Senate tax panels access to tax records. If that is proved true, the IRS whistleblower complaint could deepen concerns that the Trump administration has sought to exempt the president from long-standing rules governing taxpayer information.

Tax experts have expressed alarm that a political appointee at the Treasury Department would ask IRS career employees about any individual’s private audit.

“Historically, tax administration is solely the domain of the IRS,” said John Koskinen, a former IRS commissioner who served in the Obama and Trump administrations. “It’s an important policy that the administration of the tax code is nonpartisan, treat everybody fairly, and not have any outside interference.”

But little information about the whistleblower’s complaints have been made public, in part because of federal law limiting disclosure of information regarding confidential IRS audits. Even those who have spoken with the whistleblower have revealed very little.

“I am aware of public reports of a whistleblower complaint related to the mandatory audit program of the president and vice president,” Wyden said in a statement. “Because any discussion of this matter may implicate section 6103 privacy requirements or whistleblower protections, I cannot comment further on the matter.”