WASHINGTON - The Treasury Department's acting inspector general has opened an investigation into whether the Trump administration acted improperly during its ongoing fight with House Democrats over releasing President Donald Trump's tax returns.

Treasury Secretary Steven Mnuchin has refused to comply with a request from House Ways and Means Committee Chairman Richard Neal, D-Mass., for six years of the president's business and financial returns. Democrats have said a 1924 law explicitly gives them the authority to request the documents, but Mnuchin has denied the request and now the matter is pending in federal court.

A focus of the fight is the mandatory audit program that the Internal Revenue Service conducts on the president's and vice president's tax returns.

As the fight has intensified, an IRS whistleblower in July filed a complaint with lawmakers and relayed concerns that at least one Treasury Department official attempted to interfere in that audit process.

On Monday, Neal sent a letter to Richard Delmar, Treasury's acting inspector general, and asked for an investigation.

"I want to be assured that Treasury, including the [IRS], 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 in the letter.

Asked if the investigation encompassed the whistleblower complaint, Delmar in a brief interview referred instead to Neal's letter and said the investigation would focus on matters the lawmaker raised.

"Chairman Neal has asked Treasury OIG to inquire into the process by which the Department received, evaluated, and responded to the Committee's request for federal tax information," Delmar said in a statement. "We are undertaking that inquiry."

The Treasury Department has two inspector generals. Treasury Inspector General for Tax Administration, a separate office, has declined to comment on whether it has opened an investigation into the whistleblower's complaint.

Trump has broken decades of precedent by refusing to reveal his tax returns. He has given a variety of explanations as to why he won't make them public, saying that they are under audit or that they are too complicated for people to understand.

Democrats have filed suit against the Treasury Department, alleging Trump's tax returns must be turned over under federal law.

Last month, the Justice Department urged a federal judge to throw out the suit, saying the federal court lacked jurisdiction under the Constitution to hear the case.

"For nearly two hundred years, the Legislative Branch never invoked the power of the Judiciary to decide which side should win in a political battle with the Executive," Justice Department lawyers argued in the filing.

Neal had previously argued in a court filing that the Treasury Department, in refusing to turn over the tax returns, had "mounted an extraordinary attack on the authority of Congress to obtain information needed to conduct oversight of Treasury, the IRS, and the tax laws on behalf of the American people who participate in the Nation's voluntary tax system."

This article was written by Jeff Stein and Tom Hamburger, reporters for The Washington Post.