The June 25 accident happened near Coleta, about an hour-and-a-half southwest of Rockford. Federal investigators say the helicopter's blades apparently stopped rotating just before the crash.
The National Transportation Safety Board reports that the helicopter didn't seem to have difficulty maneuvering, but it lost altitude quickly.
Thomas Berg of Mukwonago, Wisconsin, was piloting the helicopter when it crashed into a cornfield. Also killed were Berg's young daughter, Chelsey, and Michael Siegler of Lake Geneva.
Authorities say Berg only had a student certificate that limited him to flying alone or with an instructor. He was not supposed to fly with passengers.