It says criminal investigation or prosecution — neither apply. No one has identified a crime to investigate — other than the leaking of Flynn’s name, and Mueller is not a prosecutor and he is not hired as a prosecutor.
It is a criminal investigation. Several possible crimes have been identified. Lying on your SF-86 is a crime. Most people would have at least lost their security clearance for things like omitting over 100 foreign contacts. Several campaign officials illegally accepted money from foreign governments and then tried to cover it up (Flynn, Manafort).
None of this implicates Donald Trump himself, and I wish he would stop fanning the flames. But as I said, I’m not going to second-guess his political instincts. I would not be surprise if all this hand wringing about “what is Trump doing?!” turns into jubilation of “I see what he was doing! Nicely played!”
But we’ll see.