President Donald Trump during a meeting in the Oval Office of the White House, September 16, 2019.

The Treasury Department's internal watchdog is investigating how the department handled House Democrats' requests for President Donald Trump's tax returns, CNBC confirmed Friday.

Ways and Means Committee Chairman Richard Neal, D-Mass., asked acting Inspector General Rich Delmar in a Sept. 30 letter to investigate how the Treasury handled the House panel's request to hand over tax returns for Trump and his businesses.

"I want to be assured that Treasury, including the Internal Revenue Service ... is enforcing the law in a fair and impartial manner and no one is endeavoring to intimidate or impede government officials and employees carrying out their duties," Neal wrote.

Delmar told CNBC that Neal asked his office to "inquire into the process by which the Department received, evaluated, and responded to the Committee's request for federal tax information."

"We are undertaking that inquiry," Delmar said.

The inspector general's inquiry puts more pressure on Steven Mnuchin's Treasury Department, which has resisted the Ways and Means Committee's attempts to get the tax returns through subpoenas and an ongoing lawsuit.

A spokesperson for the Treasury did not immediately respond to CNBC's request for comment.

Trump defied decades of tradition among modern presidents by failing to release his tax returns, as he had promised, either before or after the 2016 presidential election. He has claimed that he plans to release them following the completion of an audit, though there is no legal barrier to disclosing tax returns under an audit.