John Martin Tester, 41, of St. Paul, Minn., and his daughter Mikayla Nicole Tester were found dead over the weekend in a rented car parked on the side of a road in Alden Township.
The Polk County Sheriff's Department says authorities have found a note in the car listing people who should be notified about the deaths.
They also found a letter, which appeared to have been written by Tester to his four brothers saying he couldn't go through another court battle with his ex-wife regarding money and reduced visitation of Mikayla.
He also placed notes on most of his possessions, specifying what should be done with them.
Polk County Sheriff Ann Hraychuck and other authorities have said that custody issues were pending and that child support payments recently had changed.