It is widely believed that when Robert S. Mueller III testifies before Congress tomorrow, he will follow long-standing Justice Department rules against disclosing evidence regarding the conduct of people not charged in an indictment. Mueller signaled this when he sought guidance from the department on constraints and received a response directing him to avoid testimony that exceeds information included in the public version of the report he delivered as special counsel investigating Russia’s interference in the 2016 election.

Not so fast.

One would expect Mueller, with good reason, to avoid violating grand jury secrecy or disclosing classified information or evidence in ongoing cases. The Justice Department’s guidance letter to him, however, also instructs that “Established Department policy also precludes any comment on the facts developed and legal conclusions by the Special Counsel’s Office with respect to uncharged individuals, other than information contained within the portions of your report that already have been made public.” It also directs Mueller to avoid testifying about matters that might “potentially” be privileged — confusingly, because executive privilege is meant to protect confidential conversations between a president and his advisers, not evidence already collected in a criminal investigation.

Justice Department policy does indeed prohibit discussion of the conduct of people not indicted — the department does not even use names in its indictment for people who are not charged, calling them Individual 1 and so on. The reasons include concerns about privacy and reputational damage, as those not charged with crimes don’t have the opportunity to clear themselves at trial. We saw this rationale in Mueller’s analysis of the obstruction issue in his report.

But does the rationale still apply when the person at issue is the one person in our justice system who cannot be charged? Does it make sense when the conduct was committed by the president of the United States, and the choice is between a prosecutor deviating from the policy that protects “normal” targets, who can be charged if there is sufficient evidence, and the country’s remaining in the dark about the conduct of its highest elected official?

Mueller’s commitment to following the Justice Department policy that prohibits the indictment (but not investigation) of a sitting president meant he could not indict President Trump on obstruction charges. But his report also makes clear that the investigation did not exonerate the president on obstruction charges: “… if we had confidence after a thorough investigation of the facts that the president clearly did not commit obstruction of justice, we would so state. Based on the facts and the applicable legal standards, however, we are unable to reach that judgment.”

This investigative conclusion is what has placed Mueller in uncharted territory. Our entire system of justice is predicated on an adherence to the law and the rules, including long-standing practices, in the administration of that justice. But what happens in the unique case where the subject of your investigation, or target, if you will, is the one person who can’t be indicted? Do all the usual rules remain in play, or do they need to bend?

Justice Department prosecutors are taught to protect the institution’s integrity by adhering to the law and to policy. Winning is viewed as less important than playing by the rules. Failure to abide by the rules risks destroying public trust in the integrity of the system. The thinking goes that if prosecutors come to believe that the cases they are working on are so important that they must break the rules to achieve some purpose, in the end, there may be nothing left of the system they believe in and use to protect the American people.

But the long-standing tradition that good prosecutors have adhered to may be inadequate. How can the rules that protect defendants who can be charged apply to a president who is essentially above the law?

The Constitution does provide a remedy: impeachment. And Congress’s jurisdiction to impeach is far broader than a prosecutor’s to charge. Mueller could consider only whether there was sufficient evidence to prove beyond a reasonable doubt that the president or other targets had violated specific statutes. The House, however, has broad latitude to determine whether the president has committed high crimes and misdemeanors. Precisely what constitutes a high crime or misdemeanor is left to members’ judgment. The language Mueller uses in Volume II of his report can be (and has been) construed as the referral of multiple obstruction charges for Congress to consider.

So it can be argued that Congress, not the special counsel, is the mechanism for holding the president accountable. Is that enough? Does it mean Mueller should limit his answers in accordance with the guidance he received from Justice Department?

Many prosecutors believe that there are only bad outcomes when prosecutors deviate from Justice Department practices. They point to then-FBI Director James B. Comey’s decision to publicly announce and discuss the reasons for not prosecuting candidate Hillary Clinton, which led to his announcement near Election Day that he was reopening the investigation to review newly discovered emails. Despite the subsequent statement that there was no additional evidence, this announcement, so close to the election and necessitated by Comey’s earlier decision to stray from long-standing Justice Department policy against discussing evidence in an uncharged matter, conceivably could have affected the 2016 election. With that backdrop, one can understand Mueller’s steadfast commitment to staying within the boundaries of Justice Department practices in all regards, which he was certainly already hard-wired to do.

As a former prosecutor, I can say that there is comfort in the idea that one is doing justice in a fair and evenhanded way when the same rules apply to all people and to all cases. But it is difficult, if not impossible, to avoid the conclusion that despite the perils of straying from the rules, the subject of Mueller’s testimony tomorrow is not just any person, and his investigation was not just any case.

In Volume I of the report, Mueller concluded that Russia attacked our election, reaching over a million people with its messages of support for Trump, attacks on Clinton and efforts to divide the country on social media. He concluded that property was stolen from the Democratic National Committee and made public in an effort to help candidate Trump. And although he did not find sufficient evidence to establish a conspiracy beyond a reasonable doubt, he did find that the campaign welcomed offers of assistance from Russia. Even though this did not result in criminal charges, it is highly alarming. The people of our country deserve the truth and, just as important, deserve to have the facts put in context by the person who knows them best. But where is that going to come from, if not Mueller?

Prosecutors alone have access to grand juries. And the Trump administration has demonstrated that it will take any steps it can to prevent witnesses from complying with subpoenas to testify publicly before Congress. Mueller himself acknowledged that his investigation was hampered by witnesses who refused to answer questions, who lied and who destroyed or hid evidence. The public may never learn the truth regarding the president’s conduct and what it means unless it comes from the man who ran the investigation into it.

The smart money says he will follow the rules when he testifies tomorrow. Everything we know about Robert S. Mueller III, former Marine, former U.S. attorney, former FBI director, former special counsel, tells us he is not someone who colors outside the lines. But perhaps, in this instance, it is less a matter of coloring outside of them than it is of understanding that the lines need to be redrawn in this unique situation. If Mueller’s goal in his lifelong adherence to the rules is to defend the integrity of the Justice Department and protect the rule of law and the country he loves, then the task he is reluctantly honor-bound to perform tomorrow may require one last bit of extraordinary service to country from him.