With his decision that he could not make a determination whether President Trump obstructed justice, special counsel Robert S. Mueller III hewed to Justice Department legal opinions that have not been tested in the courts and were rooted in past presidential crises.

Mueller emphasized in his first public comments Wednesday that he believed his office could not even consider whether to charge the president with a crime, guided in part by legal opinions issued by the department when two previous presidents faced the threat of prosecution.

The opinions — written in 1973 when President Richard M. Nixon faced the Watergate investigation and then in 2000 following the Starr inquiry into President Bill Clinton’s behavior — concluded that initiating criminal proceedings against a sitting president would interfere with the unique constitutional responsibilities of the nation’s chief executive.

Mueller then went one step further, citing “fairness concerns,” and determined his office should come to no conclusion as to whether the president broke the law and would have been indicted had the policy not been in place.

“The Special Counsel’s Office is part of the Department of Justice, and by regulation, it was bound by that department policy. Charging the president with a crime was therefore not an option we could consider,” Mueller said.

“Beyond department policy,” he continued, “we were guided by principles of fairness. It would be unfair to potentially accuse somebody of a crime when there can be no court resolution of the actual charge.”

Before the release of his report last month, friends and former colleagues of Mueller had predicted that he would feel bound as a Justice Department employee to follow its Office of Legal Counsel opinions.

But Mueller’s view that department policy and issues of fairness prevented him from even assessing whether the president broke the law took some constitutional scholars and Justice Department veterans by surprise.

“The fact that a president cannot be prosecuted does not foreclose a finding by a special counsel of whether a president committed a crime,” said J. Michael Luttig, a former federal appeals court judge who served in the Office of Legal Counsel from 1989 to 1991.

Even senior Justice Department officials did not appear aware of Mueller’s interpretation until he was finalizing his report. Attorney General William P. Barr told a Senate panel earlier this month that the special counsel first told department leaders at a March 5 meeting that he was not going to come to a conclusion on the obstruction inquiry.

“We were frankly surprised that they were not going to reach a decision,” Barr said.

In his 448-page report, Mueller did detail potentially obstructive actions by Trump that were investigated by his team, going so far as to describe some as offering “substantial evidence” to support the elements of a criminal charge.

But the special counsel stopped short of drawing a conclusion, writing that “while this report does not conclude that the President committed a crime, it also does not exonerate him.”

Barr stepped into the void, saying he concluded that the evidence Mueller’s team gathered did not establish that Trump obstructed justice.

The legal opinions that guide the Justice Department’s current policy that a sitting president cannot be indicted have been the subject of significant debate in the legal community for years — in part because they have never been tested in the courts.

“The opinions are not nearly as clear as people suggest they are,” said Jonathan Turley, a law professor at George Washington University, who said there has been a long-standing dispute about whether the Constitution truly prohibits the president’s indictment.

Turley has argued that the analysis in the Justice Department guidelines is wrong, saying that neither the Constitution nor writings by its framers refer to a desire to make the president immune from criminal prosecution. Other scholars, he noted, believe a prosecution would intrude on the president’s ability to fulfill his constitutional duties.

The department first considered the question of a possible criminal indictment of a president in 1973, as prosecutors were weighing both Nixon’s role in covering up the burglary of the Democratic National Committee at the Watergate and whether Vice President Spiro T. Agnew had accepted bribes at the White House.

Robert Dixon, who was then head of the Office of Legal Counsel, issued an opinion declaring that while the vice president could be indicted, the president could not. While the Constitution did not expressly extend criminal immunity to the president, he found, prosecutors needed to nevertheless seek to balance the demands of the legal system with the “special responsibilities” of the presidency.

He concluded that criminal proceedings against a president should not be allowed to proceed to the point where they would “amount to an incapacitation” of the office and that the mere indictment of a serving president would damage the office’s reputation in a way that “boggles the imagination.”

Walter E. Dellinger III, who headed the OLC in the Clinton administration, called the reasoning of the Dixon opinion “shaky,” particularly in its focus on whether a president could be put on trial, rather than be indicted and potentially face a trial after leaving office.

Besides, he noted, the Justice Department offered conflicting legal analysis in other documents written around the same time, including in legal filings in court cases related to Agnew and Nixon.

The Office of Legal Counsel next considered the issue in 2000, after Clinton was impeached by the House and acquitted by the Senate on charges of perjury and obstruction of justice.

In that opinion, written after Clinton was impeached for perjury and obstruction of justice, the Justice Department concluded that indictment or prosecution of a president would “impermissibly undermine the capacity of the executive branch to perform its constitutionally assigned functions.”

Mueller cited the 2000 opinion on the first page of the volume of his report explaining his office’s approach to assessing whether Trump committed obstruction of justice.

He also wrote that out of concerns for “fairness,” his office would take an even more conservative approach and not even reach a conclusion as to whether Trump’s actions broke the law.

“Those were the principles under which we operated,” Mueller said Wednesday. “And from them, we concluded that we would not reach a determination one way or the other about whether the president committed a crime.”

Turley said he had “never heard anyone make this argument.”

“It never occurred to me that Mueller would even consider those memos as limiting his ability to reach a conclusion,” he said.

But Dellinger — who has also long argued the Justice Department opinions are flawed — said he believed Mueller’s approach was logical. After all, as Mueller noted Wednesday, the opinions also envision a process “other than the criminal justice system to formally accuse a sitting president of wrongdoing.”

“Mueller walks up to the final step of final conclusions but says, ‘That’s the role of Congress or a prosecutor able to seek an indictment. That’s their job. I’ve done my job,’ ” Dellinger said, adding: “He has handed Congress everything they need to proceed with an impeachment inquiry.”

Devlin Barrett, Alice Crites and Tom Hamburger contributed to this report.