The question on nearly everyone's mind during James Comey's testimony Thursday seemed quite straightforward: Did it suggest that the president committed a crime? But as it turns out, whether there's evidence that President Donald Trump committed a federal crime — regardless of whether it's obstruction of justice or, as he once joked, shooting someone "in the middle of Fifth Avenue" — it may not resolve what happens next.

Because he's the president of the United States. And it's not at all clear that you can prosecute the sitting president of the United States.

Sooner rather than later, this conundrum may land on the desk of special counsel Robert S. Mueller III, who has been appointed to handle the ongoing investigation into Russian efforts to interfere in the 2016 election. Comey himself said as much Thursday, that it wasn't up to him to decide obstruction of justice: "That's Bob Mueller's job to sort that out."

Mueller's selection has drawn bipartisan praise — an unusual feat in today's political climate — and, as his former FBI chief of staff, I know that he is a towering law enforcement figure with impeccable credentials and a well-earned reputation for integrity.

Mueller's stature is important precisely because he will not enjoy the same degree of legal independence from the Justice Department and the executive branch as did Ken Starr and other independent counsels under the now-expired statute that used to authorize such prosecutors. Nonetheless, if Mueller ultimately seeks to bring criminal charges against anyone other than the president, it would be politically and legally difficult for anyone at the White House or the Justice Department to stand in his way. By the same token, if Mueller decides to close the investigation without charges, both parties would recognize that as a sign the investigation did not support bringing a case.

But there is an important limitation on Mueller's activities, no matter how well regarded he is: The Justice Department's Office of Legal Counsel (OLC), the executive branch's most authoritative in-house lawyer, has long taken the position that the president cannot be prosecuted or even indicted while still in office.

First in 1973 with President Richard M. Nixon, and then again in 2000 with President Bill Clinton, the OLC determined that the indictment or prosecution of a sitting president "would be unconstitutional because it would impermissibly interfere with the President's ability to carry out his constitutionally assigned functions." Despite its Nixon-era origins, the theory is not that the president is above the law, but rather that any criminal case must wait until after he or she leaves office.

The issue is one of separation of powers. Although the Constitution sets out a mechanism by which Congress may remove the president — the impeachment process — any attempt to prosecute the commander in chief before he or she leaves office would, in the OLC's view, constitute an unworkable intrusion into the president's core responsibilities. Both in 1973 and 2000, the OLC analysis noted that the presidency is unique because the executive branch is ultimately led by a singular figure on call and on the job 24 hours a day, unlike Congress or the judiciary. If one or more members of the legislative or judicial branches are temporarily distracted, others on the job can step in to keep business going.

Not everyone agrees with the OLC's view of the law. Hofstra law professor Eric Freedman, for instance, has argued that the office's interpretation "is inconsistent with the history, structure, and underlying philosophy of our government, at odds with precedent, and unjustified by practical considerations."

The government's position has never been tested in court — although it came close during Watergate. Leon Jaworski, the special prosecutor who stood in Mueller's shoes then, argued before the Supreme Court in 1974 (in contrast with the position of the Justice Department) that "[i]t is an open and substantial question whether an incumbent President is subject to indictment."

But Mueller, who has spent nearly his entire career working for the Justice Department, seems unlikely to confront the office's conclusion head-on, given its role as the executive branch's foremost legal authority.

And that leaves a gray area: If Mueller closes his investigation without bringing charges against Trump, does that mean that evidence might exist of a presidential crime left unprosecuted? And what would happen next? As of now, it is anybody's guess.

This conundrum — a president who might very well be above the normal legal system while in office — creates a potential set of problems that Congress should anticipate and address on a bipartisan basis before it is known where this investigation (or future investigations of this president or his successors) will lead.

At least where Trump is concerned, if Mueller decides not to bring charges, that might not bring the closure all sides are expecting from his investigation unless it is accompanied by an assurance that any evidence that the president may have broken the law (if such evidence is found) will be turned over to Congress.

At present, we don't have that assurance. In fact, we don't even know that Mueller will turn over anything of value to Congress at the end of his investigation.

Justice Department regulations require a special counsel at the conclusion of an investigation to provide the attorney general (for Mueller's purposes, Rod J. Rosenstein, who is acting attorney general for this investigation, given Attorney General Jeff Sessions's recusal) with a "confidential report" that explains the decision to pursue or decline prosecution. Those regulations also require the attorney general to report to senior members of the House and Senate Judiciary committees when a special counsel concludes an investigation.

But although the rules state that these reports should include "an explanation" for the special counsel's decisions, they do not say that those reasons must be made public — nor that the reports must be detailed enough to allow Congress to take over the investigation. As for the special counsel's underlying report, the rules suggest it would never see the light of day.

Keeping a special counsel's report confidential makes sense in many circumstances. When a special counsel pursues criminal charges, then those charges should be proven in front of a jury and not preemptively described in detail to the public. If a special counsel concludes no crime has been committed — or that there is insufficient evidence to prove a crime beyond reasonable doubt — then the Justice Department's policies generally dictate that prosecutors should say no more. (The breach of that tradition and those policies, of course, was prominently cited by Rosenstein in his memo regarding Comey's firing as FBI director.)

But what if a special counsel's report is about the president himself? And what if there is speculation that the investigation found illegal activity but the special counsel didn't pursue prosecution because of the OLC's guidance? The natural body to hand off that report to would be Congress, which would have to effectively step into the shoes of a prosecutor — a point the OLC also made clear in its 2000 opinion, when it observed that a president is not rendered "above the law" because "a sitting President who engages in criminal behavior" is "subject to removal from office upon impeachment."

Congress would do well to let the investigation proceed with minimal interference. But congressional leaders of both parties should also try — in advance — to get the administration to confirm that, in the event Mueller yields potential evidence of criminal conduct by Trump, that evidence will be disclosed in detail to Congress once the investigation is concluded.

Without an agreement now, that decision will presumably be up to Rosenstein, who under the regulations will receive Mueller's report. If Congress subpoenas the report, Rosenstein could claim that it is privileged — leaving Congress and the executive branch to fight things out in a lengthy court battle. Alternatively, however, Rosenstein could commit to waiving any privileges over the report, at least to the extent that the material concerns potential criminal activity by the president. This way, Congress can be assured that it will be able to take up its own constitutional responsibilities if any evidence of criminal wrongdoing is found. And, conversely, it will know whether the president has broken no law, and members could verify that fact to the broader public.

The special counsel rules are a good, if imperfect, solution to a complicated set of legal, ethical and even constitutional dilemmas — and Mueller is the perfect candidate to fill that role. But it's a role that is more limited than many understand. Ultimately, only Congress can hold a sitting president accountable and only Congress can fully clear the record if no wrongdoing occurred.

For the one person who cannot be prosecuted in court, we must be sure that there is a clear and agreed-upon way to hand off the investigation, letting Congress follow Mueller's investigation — wherever it ultimately leads.

— Carlin is a former assistant attorney general for national security. He is currently chair of law firm Morrison & Foerster's global risk and crisis management group and also serves as chair of the Aspen Institute's cybersecurity and technology program.