The appointment of Mueller -- who is widely respected on both sides of the aisle -- comes after growing outcry, mostly from Democrats, amid fallout of President Trump's firing of FBI Director James Comey last week. While the White House initially insisted the dismissal was precipitated by Comey's mishandling of the investigation into Democratic presidential nominee Hillary Clinton's emails, Trump later told NBC's Lester Holt that the Russia investigation factored into his decision.
Two sources close to Comey told NPR that Trump asked Comey to scuttle an investigation into former national security adviser Michael Flynn the day after he resigned. Flynn stepped down after reports he had misled Vice President Pence about his conversations with the Russian ambassador. The White House denies that Trump made the request to shut down the probe.
A Comey associate says Comey wrote a memo after his encounter with the president, and congressional committees investigating the Russia ties have requested to see the memo and other related documentation. The House Oversight and Government Reform Committee has also asked Comey to testify next week.
Rosenstein, deputy attorney general, had the authority to appoint a special counsel since Attorney General Jeff Sessions has recused himself from any part in the investigation. Sessions said in March he would step back, after reports he had met twice with the Russian ambassador during the campaign — contacts he insisted were innocent.
As special counsel, Mueller will have full authorization to direct the Russia investigation, including any links or coordination between the Russian government and individuals associated with Trump's campaign, and any other matters. He can only be fired by Rosenstein, only with cause, and with notice to Congress.
Mueller, who preceded Comey at the FBI, had a 12-year stint at the agency after being unanimously confirmed by the Senate in 2001 — just the week before the Sept. 11 terrorist attacks. His tenure leading the bureau were defined by what he did to adapt to the new threats posed to America by terrorism, and he was asked by President Barack Obama to stay on beyond the typical 10-year term for two additional years, which Congress approved.
"I had been a prosecutor before, so I anticipated spending time on public corruption cases and narcotics cases and bank robberies and the like, and Sept. 11 changed all of that," he told NPR in 2013 as he was stepping down.
Mueller moved 2,000 agents from criminal investigations into counterterrorism and national security in his time leading the FBI. Airline-related terrorism was a theme for Mueller throughout his career even before that, however; as a prosecutor he spent years working on an investigation into the bombing of Pan Am Flight 103, which exploded over Lockerbie, Scotland, in 1988.
Since retiring from the FBI in 2013, he has been working as a partner for the law firm WilmerHale. The Justice Department says he will resign from the firm to avoid any conflicts of interest.
House Minority Leader Nancy Pelosi said she was "pleased" that the Justice Department had decided to appoint a special counsel, and called Mueller "a respected public servant of the highest integrity."
Senate Judiciary Committee Ranking Member Dianne Feinstein, D-Calif., called Mueller "a fine U.S. attorney, a great FBI director and there's no better person who could be asked to perform this function. He is respected, he is talented and he has the knowledge and ability to do the right thing."
Senate Majority Leader Mitch McConnell said that Mueller's appointment "confirms that the investigation into Russian intervention into our election will continue, as stated last week by Acting FBI Director Andrew McCabe. The Senate Select Committee on Intelligence will also continue its investigation into this matter."