In May 2017, Robert Mueller addressed a group of graduating students at Tabor Academy in Massachusetts. He spoke in flat tones and used direct language.

“Whatever we do, we must act with honesty and with integrity, and regardless of you chosen career, you are only as good as your word,” he told them.

“You can be smart, aggressive, articulate and indeed persuasive, but if you are not honest, your reputation will suffer, and once lost a good reputation can never, ever be regained.”

Mueller’s words are newsworthy not as much for their content, but that he spoke them at all. Since he was appointed special counsel following Donald Trump’s firing of FBI chief James Comey, they appear to have been the only words he has uttered in public.

Mueller’s long-held preference, and an approach he stuck to as be investigated Russia’s alleged interference in the 2016 election and possible collusion with Donald Trump’s campaign – a task now concluded, attorney general William Barr announced on Friday – has been to let his work speak for him.

Mueller investigation: The key figures Show all 12 1 /12 Mueller investigation: The key figures Mueller investigation: The key figures Robert Mueller is the special counsel overseeing the investigation into Russia's meddling in the 2016 election, and potential obstruction of justice by the president. Mr Mueller has a pristine reputation in Washington, where he was previously in charge of the FBI. Throughout his investigation, he and his team have been notoriously tight lipped about what they know and where their investigation has led. REUTERS Mueller investigation: The key figures Former FBI director James Comey was the catalyst that led to the appointment of special counsel Robert Mueller. Mr Comey was fired by the president after Mr Trump reportedly asked him to drop his own Russia investigation. Mr Trump has long maintained that the investigation is a "witch hunt". AFP/Getty Images Mueller investigation: The key figures Deputy attorney general Rod Rosenstein had authority over the special counsel investigation for much of the two years it has been active. Mr Rosenstein found himself with that responsibility after then-attorney general Jeff Sessions recused himself from that oversight. AP Mueller investigation: The key figures Attorney general Jeff Sessions's decision to recuse himself from oversight of the special counsel investigation may have cost him his job in the end. Mr Sessions resigned last year, after weathering a contentious relationship with Donald Trump who vocally criticised his attorney general for taking a step back. Mr Sessions recused himself from the oversight citing longstanding Justice Department rules to not be involved in investigations overseeing campaigns that officials were apart of. AP Mueller investigation: The key figures Attorney General William Barr is currently responsible for oversight of the special counsel investigation. Mr Barr's office will be the first to receive the Mueller report when it is finished. His office will then determine what portion or version of that report should be delivered to Congress, and also made public. EPA Mueller investigation: The key figures Michal Cohn is the president's former personal lawyer, who has been helping the special counsel investigation as a part of a plea deal over financial crimes, and campaign finance crimes, he has pleaded guilty to. Among those crimes, Cohen admitted to facilitating $130,000 in hush money payments to adult film actress Stormy Daniels during the 2016 campaign. Cohen has said he did so at the direction of Mr Trump. Cohen has also admitted that he maintained contacts with Russian officials about a potential Trump real estate project in Moscow for months longer than Mr Trump and others admitted. The talks continued well into 2016 during the campaign, he has said. AP Mueller investigation: The key figures Stormy Daniels has alleged that she had an affair with Donald Trump in 2006, soon after Melania Trump gave birth to Baron Trump. The accusation is of particular importance as a result of the $130,000 hush money payment she received to keep quiet about the affair during the 2016 campaign. AP Mueller investigation: The key figures Paul Manafort was Donald Trump's former campaign chairman. Manafort was charged alongside Rick Gates for a slew of financial crimes, and was convicted on several counts in a Virginia court. He then pleaded guilty to separate charges filed in a Washington court. Manafort has been sentenced to just 7.5 years in prison for his crimes — in spite of recommendations from the special counsel's office for a much harsher sentence. AP Mueller investigation: The key figures George Papadopoulos was one of the first individuals associated with the Trump campaign to be charged by the Mueller probe. He ultimately received a 14 day prison sentence for lying to investigators about contacts he had with Russian officials. AP Mueller investigation: The key figures Roger Stone is a well known political fixer and operative, who has made a name for himself for some dirty tactics. He has been charged by the Mueller probe earlier this year, and he has been said to have had prior knowledge that WikiLeaks planned on publishing stolen emails from the Hillary Clinton campaign in 2016. Getty Images Mueller investigation: The key figures Rick Gates was charged alongside former Trump campaign chairman Paul Manafort for a range of crimes. Gates, who worked alongside Manafort for a pro-Russia Ukrainian political party. The two were charged with conspiracy and financial crimes. Gates pleaded guilty. AP Mueller investigation: The key figures Former national security adviser Michael Flynn was one of the first casualties of the Russia scandal, and was forced out of his position in the White House weeks after Donald Trump took office. Flynn pleaded guilty in 2017 to "willfully" making fraudulent statements about contacts he had with Russian officials including former Russian ambassador to the US Sergey Kislyak. Flynn then lied to Vice President Mike Pence about that contact. REUTERS

Indeed, had Tabor Academy – where the veteran law enforcement official’s granddaughter was among those graduating – not secured his agreement to speak before deputy attorney general Rod Rosenstein asked him to serve as a special counsel, it is unlikely he would have uttered a word.

Mueller, say those who have known him the best, has never sought the spotlight. The work, and a sense of service, were always rewards in themselves.

“One of the reasons I went into the Marine Corps was because we lost a very good friend, a Marine in Vietnam, who was a year ahead of me at Princeton,” he said in a rare interview in 2002. “There were a number of us who felt we should follow his example.”

It is hard to imagine a more high profile or high pressure investigation than the one Mueller and his team have just competed. Indeed, in recent history observers say, perhaps only Watergate, the Iran-Contra Affair, and the sprawling Whitewater probe headed by Kenneth Starr and led to the impeachment of Bill Clinton, are on a par.

“It was inevitable that a special counsel probe into possible coordination between the president’s campaign and a foreign government, particularly when allegations of Russian interference emerged during the campaign itself, would be a high profile and controversial endeavour,” Bradley Moss, a Washington-based national security lawyer, told The Independent.

“It was made more high profile not only by consistent media coverage, but also by the circus of Trump world figures testifying before Congress, leaks about who was interviewed by the special Counsel, and the president’s regular and ongoing twitter tirades”.

He added: “The cherry on top, is that the results could, depending on the details, potentially lead to the impeachment and conviction of the president.”

At this point, we still do not know what Mueller concluded about the actions of Trump and his closest aides, other than that he has not recommended further indictments. Even now, Mueller has managed to be one of the rarest things in the nation’s capital – somebody who has not leaked.

One reason for this was the measures enacted by Mueller. Reuters said that each day members of his team arrived for work, they were made to place their mobile phones in a locker outside of the office suite.

The whole of the suite in southwest Washington had been designated a Sensitive Compartmented Information Facility (SCIF), intelligence speech for an area that has restrictions to ensure secret information stays secure, it said.

Then, there has been the personal bearing of the special counsel.

Robert Swan Mueller, 75, was born into a live of privilege, and was captain of the soccer, hockey and lacrosse teams at private school before studying at both Princeton University and New York University. He then joined the Marine Corps and served as an officer in Vietnam. Aged just 22, he was wounded in action and was awarded the Bronze Star and the Purple Heart.

As a lawyer, Mueller served in private practice before becoming a government prosecutor. In Massachusetts he pursued major financial fraud, corruption and terrorism, before being appointed the United States assistant attorney general in charge of the department of justice criminal division. In that post, he oversaw the prosecution of the Lockerbie coming and the Gambino crime family boss John Gotti.

Donald Trump launches furious tirade against Mueller on White House lawn as he calls Comey a 'bad cop'

“I have always loved investigations,” Mueller said in the same 2002 interview. “I love everything about investigations. I love the fingerprints and the bullet casings and all the rest that comes along with doing that kind of work.”

In 2001, George W Bush nominated Mueller to head the FBI. He was sworn into the job a month before the attacks of 9/11. He served in the role for ten years, before Barack Obama him to stay on for another two when he became president. When he retired in September 2013, he was replaced by former assistant attorney general James Comey, whose firing by Trump on May 9 2017 led to his appointment as special counsel a week later.

Throughout his time as special counsel, he Mueller has been a study in moderation and professionalism, choosing to avoid the media. In this, the man who has become known for combining tailored Brooks Brothers pinstriped suits with a $35 Casio DW-290 watch, has stood in sharp contrast to Starr, the flamboyant Republican lawyer who probed Clinton and helped ensure the events became a circus.

“From all appearances, Mueller has conducted his investigation with dignity, speed and intense focus. There have been no identifiable leaks out of his office, and the only way we have learned anything directly from his team has been through indictments and legal filings,” said Mr Moss.

Kathleen Clark, professor of law at Washington University in St Louis, said she believed one stark contrast between the investigations carried out by Starr and Mueller, was that the Russia probe was headed by a career prosecutor. He also chose prosecutors for the department of justice for his team. She said Mueller’s performance compared “very favourbly” to that of Starr.

Mueller faced pressure from almost the moment he started work. He and his team of lawyers have interviewed dozens of White House officials and members of Trump’s campaign team and ploughed through thousands of documents.

He also received written answers from the president, who has not only repeatedly denied there was any collusion, but attacked the investigation as a witch hunt. On numerous occasions, there has been speculation Trump was about to shut down the probe.

In the summer of last year, Trump attacked Jeff Sessions, then the attorney general on Twitter.

“This is a terrible situation and Jeff Sessions should stop this Rigged Witch Hunt right now, before it continues to stain our country any further,” said the president, who fired Sessions last November. “Bob Mueller is totally conflicted, and his 17 Angry Democrats that are doing his dirty work are a disgrace to USA!”

Mueller’s work is complete. A small team of prosecutors will remain in their positions for a short term. Mueller will be able to go back to doing whatever he was before all of this.

Instead, the focus will shift to a fight over how much of his report is made public. Mr Barr has said he wants to make available as is legally possible. At the same time, even as Democrats demand the entire report be published, the White House is preparing to invoke executive privilege to prevent the disclosure of any confidential conversation the president had with aides.