Don McGahn does not have testify before Congress, according to a memo released Monday from the Department of Justice.

The memo, released by the department's Office of Legal Counsel and addressed to White House Counsel Pat Cipollone, asserted that Capitol Hill does not have the power to make the former White House Counsel talk to them about his work for President Donald Trump.

“The immunity of the President’s immediate advisers from compelled congressional testimony on matters related to their official responsibilities has long been recognized and arises from the fundamental workings of the separation of powers,” reads the memo, signed by OLC chief Steven Engel, a Trump appointee. “Those principles apply to the former White House Counsel.  Accordingly, Mr. McGahn is not legally required to appear and testify about matters related to his official duties as Counsel to the President.”

The release of the memo provides yet another indication that the former White House Counsel and star witness in the Mueller report is unlikely to appear before the House Judiciary Committee for a scheduled interview on Tuesday.

That appearance had already seemed improbable, given that the White House had already directed him to refuse to turn over documents related to the Mueller probe. But a formal refusal to testify would add even more fuel to an already bitter political and legal fight between the parties over the boundaries of executive privilege and presidential power.

McGahn was the White House’s top lawyer for the first two years of the Trump administration, advising the president on how to navigate Special Counsel Robert Mueller’s investigation. He sat for more than 30 hours of interviews with the Mueller team—with the blessing of other White House lawyers—laying out in detail conversations where Trump asked him to interfere in the Mueller probe. Mueller ultimately chose not to decide whether or not to charge the president with obstruction of justice, but outside observers who argue he should have brought the charge point to McGahn’s statements as evidence that Trump may have broken the law.

According to the Mueller report, McGahn said that the president twice pushed him to have the special counsel fired, and also urged him to deny contemporaneous reports that he had asked him to do so. McGahn told Mueller he refused all those requests. The report also says that shortly after then-Attorney General Jeff Sessions recused himself from overseeing the Russia probe, Trump asked McGahn to “talk to Sessions” about revisiting the decision. When McGahn said he wouldn’t do that, the president “screamed” at him, per ex-White House advisor Steve Bannon’s testimony to Mueller.

McGahn is the report’s most-cited witness, by The New York Times’ count––157 times.

After the report came out, the White House still turned to McGahn for reputational clean-up. According to The Wall Street Journal, Trump asked McGahn to publicly say he didn’t think the president obstructed justice.

And Trump has publicly criticized his former confidant. “I was NOT going to fire Bob Mueller, and did not fire Bob Mueller,” he tweeted the day after the Journal story broke. “In fact, he was allowed to finish his Report with unprecedented help from the Trump Administration. Actually, lawyer Don McGahn had a much better chance of being fired than Mueller. Never a big fan!”

McGahn’s refusal to appear before the House Judiciary Committee is the latest episode in the escalating fight between members of that committee and the Trump administration. Earlier this month, Attorney General Bill Barr defied a subpoena from House Judiciary. And a top official in the department’s Civil Rights Division also ignored a subpoena to answer questions about the addition of a question on citizenship to the 2020 census.