Source: Rosenstein told lawmakers and staff he plans to allow FBI investigation to move forward, uninhibited by pressures from White House
https://twitter.com/stevebruskCNN/status/863131418460905472
A good way to do that would be to appoint a special prosecutor.
Saying "I don't plan to interfere but I reserve the right to interfere if I have to, like when I helped the president fire the FBI director," is not really all that compelling.