Attorneys for the girls are expected to ask a Waukesha County judge Monday to adjourn the case to a later time or place and to suppress some statements.
The teens are charged with attempted first-degree intentional homicide. They face trial in adult court Oct. 15.
Investigators say they plotted to lure Payton Leutner into the woods after a sleepover in Waukesha, about 20 miles west of Milwaukee. They say the girls intended to kill Leutner to please a fictional online character Slender Man.
The Associated Press has not identified the girls because the case could return to juvenile court, where proceedings are closed.