Steven Mnuchin, Secretary of the Treasury of the United States of America, during a G-20 meeting on June 08, 2019 in Fukuoka, Japan.

The Justice Department on Friday supported Treasury Secretary Steven Mnuchin's argument against turning over President Donald Trump's tax returns.

In an opinion signed by Steven Engel, assistant attorney general for the Office of Legal Counsel, the agency agreed with Mnuchin's determination that the House Ways and Means Committee's subpoena did not have a "legitimate legislative purpose."

"The Committee's asserted purpose—to consider legislation regarding the IRS's practices in auditing presidential tax filings—was implausible," Engel wrote in the memo.