Prosecutors say Anissa Weier and Morgan Geyser lured their friend into a Waukesha park and stabbed her 19 times in 2014. All three girls were 12 years old at the time. Weier and Geyser told police they committed the act to appease the fictional character Slenderman. Weier pleaded guilty in August to attempted second-degree homicide, but maintains she's not responsible due to a mental disease or defect.