WASHINGTON — The Treasury Department’s inspector general is investigating how the department handled a congressional request for President Trump’s tax returns, which Treasury Secretary Steven Mnuchin has refused to turn over.

The inquiry comes amid new concerns about political interference in the handling of Mr. Trump’s tax returns, which — like those of all presidents — are subjected to a mandatory audit. An Internal Revenue Service whistle-blower filed a complaint over the summer claiming that senior Treasury officials tried to exert improper influence over the audit. According to a government official familiar with its contents, it claims that political appointees in the Treasury Department were pressuring I.R.S. officials to ignore the requirement to scrutinize Mr. Trump’s returns.

The inspector general investigation is in response to a request from Representative Richard E. Neal of Massachusetts, the Democratic chairman of the House Ways and Means Committee, who has been leading congressional efforts to gain access to Mr. Trump’s financial information.

Mr. Trump is locked in a legal battle with House Democrats over the fate of his tax returns, and Mr. Mnuchin, one of his closest aides, has said the House demand is not legitimate because it lacks a true legislative purpose.