HIS painstaking, 15-month investigation has enraged President Donald Trump, caused consternation across America, generated numerous indictments and a thunderstorm of headlines.

Throughout it all, special counsel Robert S Mueller III, who is looking into Russian interference in the US 2016 election, possible links between Trump aides and Moscow, and potential obstruction of justice, has remained emphatically out of sight. “We haven’t had a public syllable from Bob Mueller in more than a year,” Michael Hayden, an ex-director of both the CIA and the National Security Agency, observed last week.

Instead, Mueller’s work, and that of his team of high-powered prosecutors and support staff, is speaking for itself.

An increasingly agitated Trump has worked equally tirelessly to undermine the investigation. It was a “Rigged and Disgusting Witch Hunt” that had ruined lives and was “McCarthyism at its WORST!”, he tweeted on August 19. The following day he objected that the “Disgraced and discredited Bob Mueller and his whole group of Angry Democrat Thugs” had spent over 30 hours with the White House Councel [sic]” Donald McGahn. Last summer, Trump reportedly asked McGahn to fire Mueller, but McGahn threatened to quit rather than obey.

Then, last Tuesday, came the news that Trump’s former campaign manager Paul Manafort had been convicted of eight charges of tax and bank fraud – and, much worse from Trump’s point of view, that his former attorney, Michael Cohen, had implicated Trump in directing a crime when he spoke of Trump’s role over cash payments to two women during the 2016 campaign so as to prevent them from talking publicly about their alleged affairs with Trump.

Manafort was prosecuted by Mueller’s office: details of Cohen’s activities, uncovered by the team, were passed to New York prosecutors.

“Convictions tighten squeeze on Trump,” read the main headline in Wednesday’s Washington Post. An article in the New York Times said Mueller’s probe had brought about “five guilty pleas, 32 indicted individuals, 187 charges revealing startling evidence of Russia’s 2016 attack on our democracy, and now the conviction of one of the top operators in the Trump campaign orbit”. It was “one of the most successful special counsel investigations in history”.

Some commentators drew parallels with the time in 1973 when John Dean, recently sacked as White House counsel, testified against Richard Nixon, hastening that president's downfall.

Trump’s allies have sought to smear Mueller and the inquiry, but Mueller also has many supporters, and not just in Democratic Party and liberal media circles. Republican senator John McCain, for instance, describes him as “an experienced, skilled prosecutor, and a man of exceptional probity and character”.

Mueller, 74, is the second-longest-serving director of the FBI, his 12-year tenure exceeded only by J Edgar Hoover’s near-half-century of service. Mueller took up his post the week before the catastrophic 9/11 attacks on America.

After studying at Princeton and New York universities Mueller joined the US Marine Corps and led a rifle platoon in Vietnam. One official report into a battle in a Vietnamese province in 1968 referred to his “fearless” conduct. His three years in the Corps saw him receive numerous citations and medals for valour.

Back home, he studied law and worked as a litigator before serving for 12 years in US Attorney’s Offices. In 1982, he became an assistant US attorney in Boston, investigating and prosecuting major cases that ranged from terrorist to public corruption. He then had spells as partner in law firms or in public service. In July 1990, he took over the criminal division of the US Department of Justice.

In his book Enemies: A History of the FBI, author Tim Weiner says FBI agents “instinctively liked [Mueller], despite his aristocratic demeanour ... [He] had a sharp mind, a first-rate temperament, and a high regard for well-crafted cases ... [He] was a born leader”.

One of the cases that fell to him was the investigation into the bombing of Pan Am flight 103 over Lockerbie. Weiner says that at that time, the investigation was “a surmise of supposition and surmise ... Someone needed to take charge”. Mueller quickly put FBI Special Agent Richard Marquise, who had been involved with the case from the outset, in full charge of it now, tasking him with turning intelligence into evidence.

Intelligence began to be shared much more widely and diligently, and Abdelbaset al-Megrahi was charged and convicted.

Mueller’s appointment as FBI director in the summer of 2001 came not long after he had had surgery for prostate cancer. For three years after 9/11, writes Weiner, Mueller rose before dawn, read through overnight reports, received a 7am counter-terrorism briefing and briefed President George W Bush in the White House at 8.30am. He was in charge of the biggest investigation ever.

Mueller was due to serve 10 years as director but in 2011 his term was extended by another two years by President Obama, who believed it was essential for the Bureau to have strong leadership while confronting the terror threat at home and abroad. Mueller refashioned it so it could do exactly that. He also clashed with the Bush administration on matters of principle.

He finally stepped down in September 2013, to be replaced by James Comey – who was fired by Trump in May last year.

In his own book, published earlier this year, Comey wrote that Mueller and his team were hard at work.

“The American people,” he added, “can have confidence that, unless their investigation is blocked in some fashion, they will get to the truth, whatever that is.”