Parts of the blistering 10-page letter sent to Petitti was shared with The Associated Press by a person who has seen it and spoke on condition of anonymity because they were not authorized by the school to disclose its contents.
The letter is Michigan's response to the Big Ten's notification of potential discipline of Harbaugh's undefeated second-ranked team, which is among the favorites to win the national championship.
Yahoo Sports first reported the contents of Michigan's response.
Michigan's letter said the Big Ten cannot take action if a formal decision has not been made about whether rules were violated, the person said.