According to a report in The New York Times, lawyers for Donald Trump asserted that the president can not illegally obstruct any aspect of the investigation into Russia's election meddling because the Constitution allows him to. They wrote that Trump took "severe and consequential action" in firing Flynn over his Russian Federation contacts and ordered the investigation into his actions be continued.
The memo is an assertion of presidential power and another front on which Trump's lawyers have argued that he can not be subpoenaed.
Two of Trump's lawyers at the time, John Dowd and Jay Sekulow, authored the letter in January and delivered it by hand.
The attorneys reminded Mueller that the White House could have claimed "executive privilege" over the documents, but that "the president's desire for transparency exceeded the policy purposes for the privilege under the circumstances".
Last month, Trump had targeted Mueller over spending.
If the president does not consent to an interview and Mueller instead subpoenas him, the interpretation of executive powers by Trump's lawyers would likely be tested in court if they made a decision to fight the subpoena.
He said the investigation was "so bad for our country" and asked: "Is the Special Counsel/Justice Department leaking my lawyers letters to the Fake News Media?"
'Missing' Melania Trump Briefly Surfaces To Attend White House Event
Mrs Trump made her global debut as first lady at last year's G7 summit in Italy, and went on to attend the G20 in Germany. "Thank you to the Gold Star families that joined us in celebration & remembrance", Trump wrote in the tweet .
The 20-page letter offered a sweeping assertion of the powers of the presidency as well as a detailed and robust defense of Trump's actions in dealing with the unfolding Russian Federation probe, including his firing of FBI Director James B. Comey in May 2017.
The argument, however, is not necessarily that the appointment of any special counsel would be unconstitutional, but rather that this one is, because of the broad investigative powers given to Mueller.
Mr Trump's comments on Twitter came a day after lawyer Rudy Giuliani played down the possibility that the president could pardon himself, suggesting he might have that authority but would be unwise to use it.
A 1974 opinion from the Justice Department's Office of Legal Opinion maintains that presidents can not pardon themselves "under the fundamental rule that no one may be a judge in his own case", though that question has never been tested before the country's highest court.
U.S. President Donald Trump's attorney Rudy Giuliani said on Sunday the president probably has the power to pardon himself but does not plan to do so. He added: "When will this very expensive Witch Hunt Hoax ever end?" Larry Tribe dismissed it as a Nixonian assertion that the president is above the law.
"I think the political ramifications would be tough", Mr Giuliani told ABC's This Week.
Fox News has obtained the letter, which offers a detailed look at the president's legal team and its thinking about whether he should submit to an interview, including the argument that under the law and the U.S. Constitution, Mueller faces a high bar in trying to prove he needs in-person answers from the president.