Staff for Sens. Charles E. Grassley (R-Iowa) and Ron Wyden (D-Ore.), the chair and ranking Democrat on the Senate finance committee, met with the IRS whistleblower earlier this month, those people said. Follow-up interviews are being scheduled to further explore the whistleblower’s allegations.

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

The whistleblower, a career IRS official, initially filed a complaint in July, reporting that he was told 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 due to the sensitivity of the complaint, which pertains to a confidential IRS audit that cannot be disclosed under federal law.