As Robert Mueller III probed whether President Donald Trump committed acts of obstruction, one person loomed large in the special counsel's inquiry: former White House counsel Donald McGahn.

The fraught relationship between the two men appeared to reach a boiling point when the president, suspicious that McGahn was leaking accounts of Trump's efforts to get the special counsel fired, called the former Jones Day partner “a lying bastard,” according to one account.

The exchange was detailed in Mueller's redacted report Thursday, which relayed the findings of the special counsel's nearly two-year inquiry into potential links between the Trump campaign and the Kremlin in the 2016 election, and whether Trump obstructed justice while in office.

Mueller ultimately did not make a “prosecutorial judgment” on whether Trump obstructed justice or attempted to do so. Instead, it was U.S. Attorney General William Barr and Deputy Attorney General Rod Rosenstein who made the determination to not prosecute Trump.

But the report also provided a window into McGahn's extensive cooperation with the special counsel's office, as Mueller's team grappled with the question of whether Trump ever obstructed justice. McGahn reportedly sat for 30 hours of interviews with Mueller, and the special counsel in his report said McGahn was a “credible witness with no motive to lie.”

McGahn was involved in several episodes that struck at the heart of the question of whether the president attempted to obstruct justice: there was the May 2017 firing of then-FBI Director James Comey, and incidents in which Trump directed the removal of Mueller.

McGahn's discussions with the special counsel appear to have been voluntary and directly provided to the special counsel's investigators, rather than a grand jury. As a result, the information he provided to the special counsel wasn't shielded from public view under grand jury secrecy rules.

Quinn Emanuel Urquhart & Sullivan partner William Burck represented McGahn, in addition to the White House former chief of staff Reince Priebus and chief strategist Steven Bannon.

Attempt to fire Mueller

The report examines Trump's efforts to direct McGahn to remove the special counsel shortly after Mueller was appointed to the post in May 2017. Based on McGahn's recollection, Trump repeatedly complained that Mueller had conflicts of interest and “prodded” the White House counsel to contact Deputy Attorney General Rod Rosenstein about the issue.

McGahn declined, and warned the president that if the he sought to make such a move himself, it would look like he was trying to “meddle in the investigation,” according to May 2017 notes McGahn's then-chief of staff Ann Donaldson provided to the special counsel.

McGahn told special counsel agents that Trump called him twice during a June weekend to insist on Mueller's removal. Those instructions prompted a disturbed McGahn to nearly resign from his post, consulting both his personal lawyer and Donaldson, and later calling Priebus and Bannon to inform them of his decision. McGahn would ultimately stay in the job for another year.

“McGahn did not carry out the direction, however, deciding that he would resign rather than trigger what he regarded as a potential Saturday Night Massacre,” according to the report.

In discussing McGahn's account, the special counsel said the former White House counsel “is a credible witness with no motive to lie or exaggerate given the position he held in the White House,” although a footnote in Mueller's report says that McGahn was initially reluctant to discuss those incidents, and only did so after “continued questioning.”

But Mueller also detailed how, after public reporting in early 2018 revealed Trump's directive to McGahn, Trump repeatedly sought to have the former White House counsel deny those reports that he sought to terminate Mueller—requests that McGahn declined.

Those efforts included Trump's personal lawyer calling Burck in January 2018 to have McGahn release a statement denying a New York Times report, and instructing two aides to confront McGahn about correcting the report. The president also personally met with McGahn and the White House chief of staff in February 2018, again to get McGahn to say he never ordered Mueller's firing.

McGahn told the special counsel that he believed Trump was “testing his mettle” over his perceived recollection of Trump's efforts to have Mueller fired.

“Substantial evidence indicates that in repeatedly urging McGahn to dispute that he was ordered to have the Special Counsel terminated, the President acted for the purpose of influencing McGahn's account in order to deflect or prevent further scrutiny of the President's conduct towards the investigation,” Mueller wrote in his report.