Even in his own domain, Robert Mueller is often silent. When witnesses arrive at the special counsel's office in southwest Washington, they are ushered through an underground parking garage and up to an austere, windowless conference room. Mueller's prosecutors do the talking. The man in charge, if he appears at all, greets visitors with a polite handshake and then retreats to a seat against a wall.

Since becoming special counsel for the Russia probe, Mueller has spoken only through his work: in the hundreds of pages of known court filings, some of which laid out Moscow's alleged plots to help Donald Trump win the presidency; in the 34 people and entities he charged with crimes this year; in the plea deals he made with Trump's former lawyer, former campaign chairman and another top campaign aide. Beyond that: Nothing. No interviews. No press conferences. No tweets. No leaks.

Mueller's silence has invited noisy speculation from partisans. To critics on the right, he is an overzealous prosecutor drunk on power and roaming beyond his mandate in a bid to drum Trump out of office. To liberals, he is a crusading hero who won't quit until he brings the President to justice. The public narrative of Mueller's investigation this year has often described its central character more as myth than man.

So it is instructive to hear friends and former colleagues talk about Robert Swan Mueller III. Not because they portray a perfect person, but because they describe a complicated one: relentless but circumspect, impatient but thorough, difficult but respected. Mueller, they say, is the kind of man who flicks the lights off and on at his home to inform guests that it's time to leave a social gathering, and who is so keenly aware of any appearance of impropriety that he won't even enter the same room as friends who are working on the other side of the Russia case. As FBI director, he twice threatened to resign over matters of legal principle, winning the standoff both times, and was infamous for eviscerating ill-prepared underlings. "If indicting his own mother was the right thing to do," says former FBI assistant director Tom Fuentes, "he would do it."

These qualities, manifest in Mueller's work this year, are why many believe he was ideally suited to oversee the biggest test to the American system of justice since Watergate. As the nation awaits Mueller's report and the political pressure intensifies, he has led the investigation with the same rigor and sense of duty that has marked his life. "He likes to follow procedures," says David Kris, former Assistant Attorney General in the national-security division of the Department of Justice. "And those procedures he sees as a safe harbor against the stormy seas of politicization."

Attendees at a House committee hearing on July 12 hoist posters depicting five men who pleaded guilty to charges brought by Mueller. Sarah Silibiger—CQ-Roll Call/Getty Images

What they do not guarantee is a clean outcome. Because where Mueller may be headed, there is no precedent or protocol. There's no rule book for what Mueller must do if he finds evidence that Trump is guilty of a crime. The justice system, all the way down to its foundational document, does not provide for a President to be prosecuted by the law-enforcement system he leads: it is a constitutional gray zone. Yet Mueller is a man who tends to see the world in black and white. "He sees no compromise to what he sees as right," says Mueller's friend Thomas Wilner. How the reflexive rule follower reacts when there are few to guide him may ultimately define this chapter in American history.

***

Trump and Mueller could hardly be more different. One created a public persona as the embodiment of gaudy capitalism; the other is a reticent patrician, driven and serious, who's devoted his life to institutions. One embodies disruption, the other consistency. One flouts the rules, the other enforces them. One is the avatar of disorder, the other the personification of order. Nothing less than core principles of American justice and self-government are at stake in their struggle.

This year that contest has turned into a brawl. On the advice of his lawyers, Trump originally submitted to the special counsel's probing, affirming the investigation's legitimacy and seeking to create an image of cooperation. But as Mueller's investigation has intensified—it has now stretched to over 80% of Trump's term—the President's patience has worn thin and his posture has changed.

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On Feb. 16, a federal grand jury in the District of Columbia returned an indictment against 13 Russians and three Russian companies. The 37-page document, signed by Mueller, read like a spy thriller. During the 2016 election, it alleged, the Russians posed as Americans to interfere in the presidential campaign. They helped apparently unwitting Trump campaign staff organize rallies in Florida. They created hundreds of social-media accounts posing as anti-immigration groups, Christian activists, supporters of the Black Lives Matter movement and the Tennessee Republican Party.

Mueller's rendering of the facts cast a shadow on Trump's win and settled whatever debate remained about the scale of Moscow's scheme. Project Lakhta, as the influence operation was called in later Justice Department documents, allegedly had been organized years earlier by Russian President Vladimir Putin via his close ally Yevgeny Prigozhin, an oligarch who started out as Putin's chef but had risen to run a vast network of mercenaries and spies. Over time, court documents would reveal that Mueller had access to Project Lakhta's accounts.

Within weeks of the February indictment, Trump's 2016 deputy campaign chairman, Rick Gates, pleaded guilty to tax- and bank-fraud charges , agreeing to testify against his former boss, Trump's onetime campaign chairman Paul Manafort. It was a blow to Trump: in one month, Mueller had laid out in damning detail how a foreign power had worked to help put him in the Oval Office, and brought a key campaign operative into the web of the investigation.

At the time, Trump's legal team was led by John Dowd, a veteran white-collar defense attorney. Dowd struck a cooperative tone with Mueller's team, turning over thousands of pages of documents and clearing the way for hours of testimony by close Trump aides, including White House counsel Don McGahn. Dowd and Trump refrained from disparaging Mueller in public, while the legal team conducted delicate negotiations over whether and how the special counsel's team could interview the President.

But as Mueller started flipping some of Trump's former advisers and squeezing others, Dowd and Trump reportedly parted ways on strategy. "The team started out with, We're going to cooperate and turn everything over," says Victoria Toensing, a lawyer who was in talks to join Trump's team around the time Dowd left in March. "They took a tough stand after they saw the direction Mueller was going."

The face of the new combativeness was Trump lawyer Rudy Giuliani, who came on in April. "The day Giuliani came onto the team was the day that the accommodation ended and the confrontation began," says Harvard Law School professor Alan Dershowitz, who has criticized the Mueller probe. Trump began hammering Mueller by name, while Giuliani went on a media blitz through the summer. The attacks played to conservatives' perceptions of an out-of-control "deep state" and pointed to findings by the department's inspector general that investigators and top officials at the FBI and Justice had behaved improperly. Mueller's approval rating—by now it was being tracked by news organizations, as if he were a candidate for office—dropped below 50% in June.

Giuliani admitted his goal was to discredit the special counsel's work before the public, regardless of the legal merits. "This case is not going to be tried before a jury," Giuliani told TIME in June. "It's not a criminal case. It's an investigation that's going to result in a report, and the issue will be what happens to that report, and public opinion is going to have a lot to do with that." In other words, even if Mueller reports evidence that the President broke the law, Trump wouldn't be impeached and then convicted by a Republican-led Senate if the Senators' constituents thought the whole thing was a hoax.

But as Trump lashed out, Mueller kept plugging away. In July, days before Trump was due to meet with Putin in Helsinki, Mueller indicted 12 Russians , including members of the GRU, the country's military-intelligence service. Citing specifics such as emails and street addresses, Mueller alleged that the Russian officials had targeted more than 300 people connected to Hillary Clinton's presidential campaign with spear-phishing attempts funded in part by a Bitcoin money-laundering operation. The emails were released at key moments during the race.

Back at home, Mueller moved closer to Trump. In August, a Virginia federal jury convicted Manafort on eight counts of financial crimes including bank fraud and tax fraud, committed before he joined Trump's campaign. In a related case, prosecutors announced a plea agreement with Trump's longtime personal lawyer, Michael Cohen. Cohen admitted on Nov. 29 that he had lied in written testimony to Congress about a Trump real estate venture in Moscow that he had discussed with the Kremlin at Trump's behest during the 2016 campaign.

Amid speculation that Trump might fire Mueller, protesters gathered at the White House on Nov. 8 to support the special counsel. Andrew Harnik—AP/Shutterstock

Mueller revealed just how much Cohen had been helping over the previous three months in a separate filing on Dec. 7. Cohen had alleged that multiple well-connected Russians had tried to contact Trump with business proposals from 2015 into 2016, using Cohen as a conduit. Cohen also indicated he had circulated his false response to Congress to others. In all, Mueller has brought charges against 36 people or entities for nearly 200 felony counts over his entire investigation. Each of the six Americans Mueller has charged so far has either pleaded guilty or been convicted, and more are likely in his sights.

By now Mueller's work has unearthed evidence of at least two potential crimes committed by President Trump—allegedly violating campaign-finance laws by conspiring with Cohen to pay hush money during the 2016 campaign to two women who said they had affairs with Trump. And Mueller may have only begun to lay out what he knows.

***

Each morning, Mueller leaves his home in a leafy neighborhood in northwest Washington and rides through the still quiet streets of the capital, arriving at his desk before much of the city has rolled out of bed. The special counsel's office is headquartered a few blocks south of the National Mall, amid an imposing collection of federal buildings. Like many ad hoc operations in Washington, the office interior is described by those who have seen it as "spartan."

Mueller's longer journey to the job was a lifetime in the making. Born in 1944 into a country at war, he was given the name of his father, Robert Swan Mueller Jr., a Princeton grad who captained a Navy sub chaser in the Mediterranean and later became an executive at DuPont. The younger Mueller grew up in New Jersey and attended high school at St. Paul's, an elite New Hampshire boarding school, where he played on the varsity lacrosse team with future Secretary of State John Kerry. Then he followed his father to Princeton, writing his senior thesis on African territorial disputes before the International Court of Justice.

In college, Mueller met David Hackett, an older lacrosse teammate who became a role model. Mueller decided that, like Hackett, he wanted to join the Marines. After graduating in 1966, Mueller married his high school sweetheart and enlisted in the Corps. The following year, Hackett was killed by a sniper during his second tour in Vietnam. Mueller has said that Hackett's death only strengthened his resolve to follow his classmate's service. The commitment provides a window into the 22-year-old Mueller's character: a young man with wealth, a new wife and no shortage of professional options chose to join the Marines during an escalating war.

The battlefield tests came quickly. In December 1968, his platoon was attacked at Mutter's Ridge, a notorious combat zone separating North from South Vietnam. During the hours-long gunfight, many men were killed. Mueller was awarded a Bronze Star with a distinction for valor for his conduct in that battle. A few months later, he was shot in the thigh during an ambush but saw the battle through with a bullet hole in his leg. When the wound healed a few weeks later, he returned to active duty.

Mueller served in Vietnam for one year, but the stint would set the course for the rest of his life. "You can't understand him without understanding his roots as a Marine," says Lisa Monaco, his former chief of staff at the FBI and later homeland-security adviser to President Barack Obama. "His leadership style and his work ethic and his focus on leading by example and with integrity are all completely bound up and informed by his service as a Marine." That Mueller survived his tour while others did not only heightened his sense of duty. "Perhaps because of that," Mueller said during a 2013 commencement address at the College of William & Mary, "I have always felt compelled to try to give back in some way."

After law school at the University of Virginia, Mueller spent three years as a litigator in a private firm in San Francisco, then 12 in the U.S. Attorney's offices in San Francisco and Boston. He joined the Boston law firm of Hill and Barlow in 1988, but learned he hated billing hours and chasing clients. In 1989 he went to work at the Justice Department, becoming head of its criminal division under Attorney General William Barr—who looks set to become his boss again soon, after being nominated for the same role on Dec. 7 by Trump. In his perch atop DOJ's criminal division, Mueller oversaw the investigations of Panamanian dictator Manuel Noriega; the bombing of Pan Am Flight 103 over Lockerbie, Scotland; and the prosecution of the mob boss John Gotti.

When Bill Clinton was elected President in 1992, Mueller—a lifelong registered Republican—left government to become a partner at the white-shoe law firm Hale & Dorr. But he bristled at representing clients he thought might be guilty, friends say, and shocked colleagues by becoming a homicide prosecutor in the U.S. Attorney's office in Washington during the crack epidemic, trading a lucrative job for a grueling, anonymous one. "This is not flashy work, it is not high-profile work, it is not work that makes you famous," says Kris, now co-founder of the Culper Partners consulting firm. "That tells you something about what's important to him."

When George W. Bush became President in 2001, he picked Mueller to become FBI director. Seven days after his swearing-in came the Sept. 11 terror attacks. For the next 12 years, under Bush and later Obama, Mueller reshaped the bureau to focus on intelligence and counterterrorism along with traditional criminal law enforcement, attacking the mission with focus and intensity.

For all his admirers, plenty of employees at the bureau disliked Mueller. Critics chafed at his overbearing style and dubbed him Uncle Bob. He chewed through special agents in charge who were slow to adapt to his demands. Even many supporters found him to be a remote leader. "There was a sense that he was a bit callous," says a former high-ranking FBI official who worked closely with Mueller, "and that he was so driven by the mission that he forgot about the people." Former Attorney General Alberto Gonzales, who met daily with Mueller for more than two years, considers him a friend but adds, "I don't remember any instances where Bob and I just sat around chatting and joking around and kidding." The seriousness was even reflected in his wardrobe: Mueller preferred to wear white shirts to work, an old FBI tradition dating back to the Hoover era.

Mueller acknowledges his impatience. In his address at William & Mary, he recalled that during his days at Justice he'd often cut off attorneys by saying, "What is the issue?" One evening, he said, his wife started telling him about a hard day. He interjected with the same curt question. She did not appreciate it. "I am your wife," he recalled her saying. "I am not one of your attorneys. Do not ever ask me, 'What is the issue?' You will sit there and you will listen until I am finished.

"That night, I did learn the importance of listening to those around you—truly listening—before making judgment, before taking action," he continued. "I also learned to use that question sparingly, and never, ever with my wife."

***

As special counsel, Mueller was charged in 2017 with uncovering "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump" as part of the "investigation of the Russian government's efforts to interfere in the 2016 presidential election." He was also empowered to pursue "any matters that arose or may arise directly from the investigation."

The big question now is whether Mueller will find evidence that the President colluded with a foreign power to get elected or obstructed the investigation. Mueller has already charged several of Trump's aides with lying to investigators. If he finds evidence that the President did commit a crime, that could in turn lead to impeachment proceedings in the House.

Mueller faces a harder challenge if he believes Trump should be prosecuted under the federal justice system. There's no definitive answer as to whether a President can be prosecuted. It's never happened before, and no court has ruled on the issue. (The Supreme Court heard arguments about it in 1974 in relation to Richard Nixon's role in the Watergate cover-up but never resolved the question.) The Constitution doesn't explicitly state whether a President can be prosecuted while in office. The official view of the Executive Branch is that it can't be done: in 2000, the Office of Legal Counsel wrote a memo arguing that a President can't be indicted.

It's not clear if Mueller has his own view of this difficult constitutional question. Nor is it clear what he thinks might be appropriate in this case. For a man devoted to protocol and committed to a black-and-white view of the world, such a moment would be the ultimate test of character. What documents will Mueller refer to if he faces that judgment? What higher principles will guide him as he sorts through his responsibilities? On that, too, Mueller remains silent. —With reporting by Molly Ball