NBC News: "In my capacity as acting Attorney General, I determined that it is in the public interest for me to exercise my authority and appoint a Special Counsel to assume responsibility for this matter," 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. What I have determined is that based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command."
Mueller led the FBI for 12 years under Presidents George W. Bush and Barack Obama. He is second only to J. Edgar Hoover for longest tenure for an FBI chief.
Mueller has agreed to resign from his private law firm in order to avoid any conflicts of interest, the AG's office said.
Related: Special Counsel Will Take Over FBI Russia Campaign Interference Investigation
The FBI, with the help of the Treasury Department, the CIA and other agencies, has been examining evidence of possible contacts, money transfers and business relationships between a variety of President Trump's associates and Russian officials, sources say.