Robert Mueller, who has been appointed special counsel to investigate possible collusion between President Donald Trump's 2016 campaign team and Russia, is a former FBI director who took over the bureau just one week before the 9/11 attacks on the US.
While the 72-year-old’s tenure went on to be defined by the country’s counter-terrorism efforts, the former federal prosecutor’s career is also marked by high-profile investigations, including the Lockerbie bombing, and a standoff with the Bush administration over domestic wire-tapping.
The appointment by the US Justice Department followed a week of turmoil for the White House amid rising demands for an independent probe of alleged Russian efforts to sway the outcome of November's presidential election in favour of Mr Trump and against Democrat Hillary Clinton.
Pressure has been building on Mr Trump since his firing of James Comey, head of the Federal Bureau of Investigation, who had been leading a federal probe into the matter.
Career as a prosecutor
After serving as a Marine officer in the Vietnam War, for which he was awarded a Bronze Star, Purple Heart and two Navy Commendation Medals, Mr Mueller worked for 12 years in the US Attorney offices.
Known for his tough, no-nonsense managerial style, he investigated and prosecuted crimes ranging from major financial fraud to public corruption cases.
In 1990, he became head of the criminal division of the US Department of Justice, where he oversaw the high-profile investigations of Panamanian leader Manuel Noriega, the Gambino crime family boss John Gotti and the Lockerbie bombing.
In 2009, he wrote a scathing letter to Kenny MacAskill, Scotland's then justice secretary, in which he condemned the decision to release Abdelbaset Ali Al Megrahi, who was convicted over the 1988 bombing of Pan Am Flight 103.
Mr Mueller said the decision made "a mockery of justice" and gives comfort to terrorists around the world.
Time at the FBI
After serving several months as Acting Deputy Attorney General in 2001, Mr Mueller was appointed the sixth head of the Federal Bureau of Investigation after being nominated for the post by President George W Bush.
He took over the post just one week before September 11 terror attacks on the US and went on to transform the bureau into a counterterrorism agency, expanding its manpower and shifting 2,000 of the 5,000 agents in its criminal programmes into national security.
During his tenure, terrorists were thwarted in their efforts to bring down a trans-Atlantic flight in 2001, a Detroit-bound jetliner on Christmas in 2009 and US-bound cargo planes carrying printer cartridge bombs in 2010.
"You sit down with victims' families, you see the pain they go through and you always wonder whether there isn't something more" that could have been done, he said in an interview in 2013.
One of the most notable episodes of his tenure came in March 2004, when both he and Mr Comey threatened to resign to stop the Bush administration from reauthorising the domestic surveillance programme secretly launched following the September 11 terrorist attacks.
Mr Comey was deputy attorney general at the time and had brought in Mr Mueller as they went to the hospital bed of the then attorney general, John Ashcroft, who was being pressured by White House officials to renew the no-warrant wiretapping programme. The justice department had ruled the programme to be illegal.
The threat of the resignations persuaded Mr Bush to restructure the programme.
Mr Mueller went on to become the longest serving director of the FBI since J. Edgar Hoover when Barack Obama asked him to extend his tenure by two years. With the Pentagon and CIA getting new leaders, the former president was keen to ensure some continuity in his national security team.
As head of the FBI, he was known for his hands-on style. "The management books will tell you that as the head of an organisation, you should focus on the vision," Mr Mueller once said. But "for me there were and are today those areas where one needs to be substantially personally involved."
After the FBI
Following his departure from the FBI, Mr Mueller taught at Stanford University, focusing on cyber-security issues, and joined the law firm WilmerHale as a partner.
Among the firm’s clients are Mr Trump’s daughter, Ivanka, and her husband, Jared Kushner.
A spokesman for WilmerHale said Mueller had “resigned from the firm immediately upon his appointment by the Deputy Attorney General.”
How has his appointment been received?
Republicans and Democrats alike have praised Mr Mueller as someone widely respected for his integrity and independence.
"I welcome and applaud the decision," said Richard Blumenthal, a Democrat sitting on the judiciary committee.
Donald McGahn, the White House counsel, was given less than an hour's notice of the appointment, it was reported.
Mueller is a great selection. Impeccable credentials. Should be widely accepted.— Jason Chaffetz (@jasoninthehouse) May 17, 2017
Mr Trump issued a short statement, saying said he looked forward to a quick resolution of the matter.
"As I have stated many times, a thorough investigation will confirm what we already know - there was no collusion between my campaign and any foreign entity," he said.
Lawmakers on Capitol Hill welcomed the Justice Department action, but House and Senate Republican leaders said they would go on with their own investigations of the Russia matter.
Former Director Mueller is exactly the right kind of individual to serve as special counsel in the Russia investigation.— Chuck Schumer (@SenSchumer) May 17, 2017
Senate Democratic leader Chuck Schumer said Mr Mueller was the right choice for the job.
“A special counsel is very much needed in this situation and Deputy Attorney General Rosenstein has done the right thing," Mr Schumer said in a statement.
Republican House Judiciary Committee Chairman Bob Goodlatte said he was confident Mr Mueller "will conduct a thorough and fair investigation."
Having known him for years, I believe special counsel Mueller is a very good thing. He is one of the best -- independent and no-nonsense. https://t.co/nMQo6aHalM— Preet Bharara (@PreetBharara) May 17, 2017
Preet Bharara, a former crusading New York prosecutor who was fired by Mr Trump in March, praised the appointment as a “very good thing”. “He is one of the best - independent and no-nonsense,” he added.