One of the two girls accused of stabbing their 12-year-old friend to please a mythical figure known as Slender Man pleaded not guilty in a U.S. court Friday on the grounds of mental illness. Morgan Geyser and Anissa Weier had allegedly stabbed their classmate 19 times in May 2014.

Two doctors have been appointed by a judge to evaluate Geyser, who is charged with attempted murder. If convicted, Geyser and Weier face decades in jail. Weier had pleaded not guilty last year.

The two girls are accused of luring a classmate into the woods following a sleepover at one of the girls' homes on May 31, 2014, for a birthday party. A passing bicyclist found the young girl in a park in a Milwaukee suburb with 19 stab wounds. She suffered serious internal injuries but managed to survive. 

Both girls, who were arrested the following day, told police they hoped to impress the fictional character Slender Man, about whom they read on the horror storytelling website, Creepypasta Wiki.

Investigators believe that the two girls had been plotting for months to kill their victim, BBC reported.

Last month, an appeals court in Wisconsin confirmed an earlier decision by a lower court that it was reasonable to try both girls, who are now 14, as adults because the crime was planned and violent.

Their lawyers argued that the case be heard in juvenile court as both teenagers were suffering from mental illness.