Senate investigators have conducted an extensive interview of a whistleblower at the Internal Revenue Service who has alleged improper political interference in the audit of the president or vice president, according to two people familiar with the meeting.

The whistleblower delivered transcribed remarks to staffers for Sens. Charles E. Grassley (R-Iowa) and Ron Wyden (D-Ore.), the chairman and highest-ranking Democrat on the Senate Finance Committee, respectively, these people said. The interview occurred in recent weeks.

The whistleblower, a career IRS employee, has alleged that at least one political appointee at the Treasury Department may have tried interfering with the audit of President Trump or Vice President Pence. Democratic lawmakers have stressed the importance of thoroughly investigating the complaint, while Trump administration officials have downplayed its seriousness and said it is based on hearsay.

The continuing investigation into the whistleblower’s allegations comes amid the Senate impeachment trial of the president and Senate Republicans’ push for his acquittal, a process kicked off by a complaint by a whistleblower in the U.S. intelligence service.

Spokespeople for both Wyden and Grassley declined to comment or confirm the interview. The two 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.

“It seems Wyden and Grassley are doing their due diligence. The tax writing committees ought to find out about this,” said Steve Rosenthal, a senior fellow in the Urban-Brookings Tax Policy Center, a nonpartisan think tank. “The next step would be, depending on what happened, pursuing the next step to corroborate what the whistleblower said.”

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, who said it raises “serious and urgent concerns” about the system for auditing the president. That process is supposed to be walled off from political actions. Neal revealed the complaint as part of his lawsuit against the Trump administration for the release of the president’s tax returns, which Trump has refused to release in a break with decades of precedent.

Neal has sought to obtain Trump’s tax returns from the IRS, which is a division of the Treasury Department, but Treasury Secretary Steven Mnuchin — who served as the national finance chairman for Trump’s 2016 campaign — has opposed releasing the records and said doing so could compromise taxpayer privacy. There is no precedent for the Treasury Department or the IRS to refuse to furnish tax records requested by the chairman of the House Ways and Means Committee. The matter is pending in federal court.

The Washington Post previously reported that the complaint 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. It is unclear whether investigators have interviewed the other IRS official or whether this person is willing to speak to House or Senate lawmakers.

There has been extreme focus on Trump’s tax returns and less scrutiny of those of Pence, but the presidential audit program applies to both offices. The Post has not 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 IRS commissioner during the George W. Bush administration.