WASHINGTON — The Justice Department has appointed Robert S. Mueller III, the former F.B.I. director, to serve as a special counsel to oversee its investigation into Russian meddling in the election, Deputy Attorney General Rod J. Rosenstein announced on Wednesday.
The appointment of Mr. Mueller dramatically raises the stakes for President Trump in the multiple investigations into his campaign’s ties to the Russians. It follows a swiftly moving series of developments that have roiled Washington, including Mr. Trump’s abrupt dismissal of the F.B.I. director, James B. Comey, and the disclosure that the president urged Mr. Comey to drop the bureau’s investigation into his former national security adviser, Michael T. Flynn.
“I determined that it is in the public interest for me to exercise my authorities and appoint a special counsel to assume responsibility for this matter,” Mr. Rosenstein said in a statement. “My decision is not a finding that crimes have been committed or that any prosecution is warranted. I have made no such determination.”
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While a special counsel would remain ultimately answerable to Mr. Rosenstein — and by extension, the president — he would have greater autonomy to run an investigation than a United States attorney would. Mr. Mueller will be able to choose to what extent to consult with or inform the Justice Department about his investigation as it goes forward.
Mr. Mueller is viewed by members of both parties as one of the most credible law enforcement officials in the country. He served both Democratic and Republican presidents, from 2001 to 2013, and was asked by President Barack Obama to stay on beyond the normal 10-year term until Mr. Comey was appointed.
Inside the F.B.I., he is known for his gruff, exacting management style — and for saving the institution. After the Sept. 11 terrorist attacks, there were calls to break up the F.B.I. and create a separate domestic intelligence agency. Mr. Mueller, who came to the agency just one week before the attacks, beat back those efforts and is credited with building the modern F.B.I. He led the investigations into Al Qaeda while simultaneously transforming the agency into a key part of the national security infrastructure.
“He’s an absolutely superb choice,” said Kathryn Ruemmler, a former prosecutor and White House counsel under Mr. Obama. “He will just do a completely thorough investigation without regard to public pressure or political pressure.”
She added: “I cannot think about a better choice.”
The order to appoint Mr. Mueller was signed by Mr. Rosenstein on Wednesday, drawing on a regulation granting the attorney general the authority to appoint a special counsel for only the second time in history. The first time that regulation was used was in 1999 by Janet Reno in the case of Jack Danforth, a former Republican senator from Missouri. He investigated the botched federal raid on the Branch Davidian compound in Waco, Tex., in 1993 that killed 76 people.
In his capacity as special counsel, Mr. Mueller will be able to request additional resources for the investigation. Those requests will be reviewed by Lee Lofthus, assistant attorney general for administration.
Mr. Mueller is expected to announce his resignation from the law firm WilmerHale.