United flight 2005 from Chicago was headed to Minneapolis but landed at the Dane County Regional Airport in Madison, Wisconsin.
"United flight 2005 from Chicago to Minneapolis landed safely in Madison, Wisconsin to address a security concern with an unruly passenger," the airline said in a statement. "The flight is expected to continue to Minneapolis later on Friday."
The Dane County Sheriff's Office said the passenger was a 75-year-old man "appeared confused and in mental health crisis."
"FBI Milwaukee's Madison Resident Agency immediately responded and is handling the investigation. They advised no criminal charges are being pursued at this time," a spokesperson for the sheriff's office said.
The sheriff's office said the passenger's family in Minnesota was contacted and will travel to Madison.
No injuries were reported, United said.