Although updates on the probe have periodically leaked to the press, Mueller has remained tight-lipped. It's unclear how close he may be to concluding the case, but some estimates say it could take at least a year.
Some Trump allies have criticized Mueller's closeness to Comey and accused him of overreaching in the investigation. In response to calls for his firing, senators introduced bipartisan legislation that would allow Mueller to challenge the Department of Justice in court if he were removed.
Sources: LA Times, New York Post, WBUR
On May 17, Deputy Attorney General Rod Rosenstein appointed Mueller special counsel to take over the Russia investigation, including "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump."
Sources: Business Insider, DOJ
In May, Trump fired FBI Director James Comey, who testified before Congress that he thought it was because of the investigation he was conducting into Trump and his associates' ties to Russia. Sessions had recommended his firing, so his recusal was then thrown into question.
Sources: Business Insider, Business Insider
In March, Attorney General Jeff Sessions recused himself from all Russia-related investigative matters after failing to disclose his campaign-trail meetings with Russian Ambassador Sergey Kislyak, setting off a chain of events.
Source: Business Insider
In 2015, NFL Commissioner Roger Goodell put Mueller in charge of investigating the widely-publicized Ray Rice incident. Rice, then a running back for the Baltimore Ravens, was indicted on third-degree aggravated assault for beating his fiancée, although the charges were later dropped.
Source: Mueller NFL Report
During his private and public sector careers, Mueller presided over many noteworthy cases, including the prosecutions of Panama's former dictator, Manuel Noriega, and New York City crime boss, John Gotti.
But it's Mueller's breadth of experience with high-profile litigation that has earned him the most respect.
Even though Mueller is a Republican, many Democrats admire him. Earlier this year, Democratic Senator Jim Himes said, "There's not anybody with as much credibility internally or whose integrity is as unimpeachable as Bob Mueller."
Sources: The Washington Post, The Washington Post
Mueller is often praised on Capitol Hill for his service under both Republicans and Democratic presidents.
After Mueller finished his 10-year term in 2011, President Barack Obama asked him to stay for two more years. He was the longest-serving FBI director since J. Edgar Hoover, who founded the bureau in 1935.
Source: FBI
In 2004, there was a standoff between Mueller and Bush after he and other Justice Department officials threatened to resign if changes were not made to the National Security Agency's domestic wiretapping program. Bush conceded in the end.
Source: Washington Post
In 2002, Mueller was the subject of lawsuits filed by Muslim immigrants who claimed they had been beaten and abused by officials in immigration detention centers because of policies the Bush Administration implemented after 9/11. The Supreme Court ruled in 2017 that Mueller, along with John Ashcroft, the former attorney general, could not be sued.
Source: The New York Times
A week before 9/11, he was sworn in as the director of the FBI under President George W. Bush. The Senate confirmed him unanimously even though he was diagnosed with prostate cancer at the time.
Working on major cases involving financial fraud, terrorism, public corruption, money laundering, and narcotics conspiracies helped Mueller become a seasoned investigator over the years.
Mueller reportedly explained his decision by saying, "There's just too many young people dying violently in this city, and I want to do my share to put an end to that."
Source: TIME
Mueller is respected among his colleagues for his dedication to public service. In one example, a former associate was stunned when Mueller chose in 1995 to work in the homicide section of the US Attorney's Office in DC, which was perceived as a demotion for the highly credentialed lawyer.
Then he spent more than a decade in government working for US Attorney offices in California, Massachusetts, and Washington, DC, gaining particular expertise in prosecuting white collar crime.
After his military service, Mueller joined the San Francisco office of the international law firm, Pillsbury, Madison and Sutro. He worked as a litigator for three years.
In 1968, Mueller enlisted in the Marine Corps and went on to become a decorated officer serving in Vietnam. He received a Bronze Star, a Purple Heart, the Vietnamese Cross of Gallantry, and two Navy Commendation Medals.
When one of his friends died in the Vietnam War, Mueller was inspired to join the military. He had been previously pursuing a career in the medical field.
Source: Princeton Alumni Weekly
Mueller went to undergrad at Princeton University, got his Master's in international relations from New York University, and graduated from the University of Virginia School of Law in 1973.
Source: FBI
Born Robert Swan Mueller III in New York City in 1944, "Bob" grew up in the suburbs of Philadelphia, the elder brother to four younger sisters. Their father was an executive at DuPont. He captained the soccer, hockey, and lacrosse teams in high school.
Sources: FBI, St. Paul's School
Source: Business Insider India