WASHINGTON • Mr Robert Mueller, who served as director of the Federal Bureau of Investigation (FBI) during the last two administrations, brings to his new role as special counsel a proven willingness to take on a sitting president.
In a high-drama episode in 2004, he and then Deputy Attorney-General James Comey were preparing to resign from their positions if President George W. Bush reauthorised the National Security Agency's warrantless wiretap programme without changes. Mr Bush backed down.
Now, Mr Mueller is charged with another politically fraught mission: The investigation of possible coordination between President Donald Trump's associates and Russian officials seeking to meddle in the 2016 campaign.
Former colleagues said the ex-Marine Corps officer and former US attorney, who was sworn in as FBI chief a week before the 2001 terrorist attacks, is uniquely suited to the task.
"He doesn't sway under political pressure," said Mr Thomas Pickard, who was deputy director of the FBI under Mr Mueller in 2001. He noted that President Barack Obama extended Mr Mueller's term even after he had served through all eight years of the Bush administration.
"For 12 years, he kept the FBI out of politics," said Mr Pickard.
Mr George Terwilliger, who has known Mr Mueller since both were assistant US attorneys 30 years ago, called him "a terrific choice".
"I have no doubt that he will be even-handed - including going hammer and tong after anyone who is leaking investigative or classified information," said Mr Terwilliger, who served as deputy attorney-general while Mr Mueller led the Justice Department's criminal division.
Mr Mueller is also known as a no-nonsense, relentless prosecutor with a deep reverence for the rule of law. "The most devastating thing that can happen to an institution is that people begin to shade and dissemble," he told Washingtonian magazine in 2008.
He also has long ties to major players in the tumultuous political story that has engulfed the Trump presidency. Deputy Attorney-General Rod Rosenstein, who tapped him for the task, served under him at the Justice Department as a criminal prosecutor. Mr Comey, whom Mr Trump fired last week, was his ally during the Bush era and succeeded him as FBI director.
Until his surprise appointment on Wednesday, he was a partner at WilmerHale, a firm that represents former Trump campaign chairman Paul Manafort, Mr Trump's daughter Ivanka and son-in-law Jared Kushner.
Mr Mueller grew up in Philadelphia and went to St Paul's School, the elite prep school in New Hampshire, where he played hockey with former secretary of state John Kerry.
At Princeton, he was inspired to join the Marine Corps by a former student who died in Vietnam, according to the Washingtonian. After graduating from the University of Virginia Law School, he worked for a dozen years as an assistant US attorney in San Francisco and Boston, where US attorney William Weld described him as a "straight arrow".
"He didn't try to be elegant or fancy; he just put the cards on the table," Mr Weld told the Washingtonian.