Prosecutors said a man killed his three daughters in River Falls last July in order to hurt his ex-wife.
The St. Paul Pioneer Press reported it's the first time authorities have revealed Aaron Schaffhausen's motive.
Schaffhausen faces three counts of first-degree intentional homicide for the deaths of 11-year-old Amara, 8-year-old Sophie and 5-year-old Cecilia. All three girls were found tucked into their beds with their throats slit.
Prosecutors offered the motive at a pretrial hearing Wednesday. Also Wednesday, a judge told Schaffhausen's attorney he has until Dec. 14 to file a motion to plead not guilty by reason of insanity, and until Dec. 28 to request a change of venue.
Schaffhausen faces mandatory life in prison if convicted. The judge could grant him a chance at parole, however.