It definitely came from Michigan.
And given that a huge public university was able to respond to a FOIA request in at most three days is a pretty good hint that Michigan dropped a little birdy by the AP office suggesting an enterprising reporter might want to file a FOIA request asking for any communications between ND and Michigan over the last week.
Of course, it's possible ND dropped a similar hint and sent the reporter Michigan's way, but if Michigan didn't want the information out so quickly, they would have slow-walked the release.
So I'm pretty sure Michigan Men leaked/provided the letter
their protestations of ignorance to the contrary. AP said they got it through a FOIA request, right? Well, ND isn't subject to FOIA since it is private -- Michigan is because it is a public institution.
That would explain why early stories had a Michigan reaction and not much from ND's side.
Did anyone see it mentioned who the FOIA request was filed with? All I saw was very unspecific, which leads me to believe the Skunkbears were trying to win the PR battle with a preemptive strike.
EDIT: If anyone has a lot of money and time on their hands, they should FOIA every email sent from the Michigan Athletic Department's press people. I've done that before in Maryland, and hilarity always ensues.