Deputy Attorney General Rod J. Rosenstein named Robert Mueller, former head of the FBI, to head the investigation. Mueller served under both Republican and Democratic presidents.

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. (NYT)