Well, they can't subpoena anyone or compell them to talk with invesitgators. They can't seize phone records or email archives. There is no allegation of a crime in any way as far as I can tell. How exactly would you suggest that a formal investigation proceed? Why would anyone involved agree to talk with anyone involved in that investigation? Why would anyone voluntarily hand over anything to ND's investigators?
I'd love it, as much as the next Michigan fan, if ND was involved in a colossal conspiracy and was found out.... But wishing it is so doesn't make it so.