Morgan Geyser and Anissa Weier, now both 13, were 12 years old at the time of the alleged crime.
If the girls are convicted in the adult court system, they could each face up to 65 years in prison.
Morgan and Anissa were arrested on May, 31, 2014, after allegedly stabbing their then-12-year-old friend Payton Leutner 19 times and leaving her in the woods in their hometown of Waukesha, Wisconsin. Payton was rushed to the hospital with life threatening injuries and survived.
Prosecutors have said that both Morgan and Anissa were obsessed with the fictional character Slender Man, who is often depicted in fan fiction stories online as a horror figure who stalks children. In court in February, disturbing drawings depicting Slender Man and dismembered Barbie dolls recovered from Morgan’s bedroom were shown.
When asked to describe Slender Man, Morgan told the detective during an interrogation that he was "a tall, faceless man who preys on children," according to an audio recording of the interrogation made public.
"He watches you," she added. "I've never seen him. He's everywhere."
Authorities said Morgan repeatedly told detectives it was "necessary" to kill Payton to become "proxies" of Slender Man and live with him in his mansion in the woods. They believed his mansion was in the Chequamegon-Nicolet National Forest in northern Wisconsin, authorities said. Morgan told police their plan to kill their friend began as early as December 2013, authorities said.
The Associated Press contributed to this report.