By choosing former FBI Director Robert Mueller to serve
, Rosenstein picked an FBI veteran with broad support on both sides of the aisle and sterling reputation. The move takes Justice Department leadership out of the equation and clears the way for Mueller to have the resources to conduct the investigation.
The code of federal regulations provides that the attorney general will appoint a special counsel when the attorney general "determines that criminal investigation is warranted and that an investigation by the Justice Department would present a conflict of interest 'or other extraordinary circumstance' and that 'under the circumstances it would be in the public interest to appoint an outside special counsel."
Because of the recusal of Attorney General Jeff Sessions, a campaign ally of President Donald Trump, the decision to name Mueller was made by Rosenstein.
Muller, however is not strictly independent. Rosenstein will still maintain the ability to fire Mueller.
"The special counsel will have all of the powers of a federal prosecutor, but he will do his work outside of the regular chain of command in the Justice Department," said Brian C. Kalt, a professor of law at Michigan State University.
Kalt says the independence is important," because it is otherwise problematic to have the government investigating itself."
In his statement, Rosenstein noted that Mueller will be able to "follow the facts, apply the law and reach a just result," which means the investigation could expand.
Indeed, the Justice Department order announcing the appointment makes clear that the special counsel is authorized to conduct an investigation including "any matters that arose or may arise directly from the investigation and any other matters within the scope" of the special prosecutor rules.
"Critics of the special counsel will worry that the investigation could mushroom, much like the Whitewater investigation led to President Bill Clinton's impeachment for lying about Monica Lewinsky," said CNN contributor Steve Vladeck of the University of Texas School of Law. "But supporters will surely argue that the benefits of impartiality out weight any such costs."
The appointment may also affect any congressional investigations. It is likely to take some pressure off of congressional Republicans who can now point to the special counsel as the only investigation that matters.
While congressional investigations could continue in parallel, Kalt said, Congress is likely to back off some to allow Mueller to do his job.
"Robert Mueller is not one of the best, he is the best," said Philip Mudd CNN's counterterrorism analyst. "There is nobody better at doggedly pursuing a target without succumbing to pressure."