By Brendan O'Brien

MILWAUKEE (Reuters) - One of the two Wisconsin girls accused of attacking a classmate to please a fictional character named Slenderman changed her plea to not guilty due to mental illness on Friday, according to court records.

Anissa Weier and Morgan Geyser were charged with attempted first-degree homicide in the May 2014 stabbing attack in Waukesha, a suburb of Milwaukee. All three girls were 12 years old at the time of the stabbing. Weier and Geyser, who will be tried as adults, are now 14. [L1N1AD1NL]

Weier changed her plea from not guilty to not guilty by reason of "mental disease or defect" in a Waukesha County Circuit Court according to online court records filed on Friday. The change of plea was entered by Maura McMahon, one of Weier's attorneys.

Judge Michale Bohren ordered that Weier be evaluated by two doctors by Oct. 6, court records showed. The next hearing for Weier and Geyser is scheduled for Oct. 13.

Geyser's plea was changed to not guilty by reason of mental disease or defect last month, according to court records.

The girls would likely be treated at a mental hospital if they are found not guilty by reason of mental disease or defect.

A Wisconsin appeals court ruled in July that the teenagers should be tried as adults after their attorneys tried to get the case moved to juvenile court. [L1N1AD1NL] They can appeal that decision to the Wisconsin Supreme Court.

Weier and Geyser could each be sentenced to up to 65 years in prison if convicted as adults. They could be held until the age of 25 if convicted as juveniles.

Wisconsin law requires cases to begin in adult court if they involve juvenile suspects at least 10 years old who are charged with first-degree attempted intentional homicide.

The girls' lawyers had argued the girls were mentally ill when they stabbed their classmate. A judge ruled Weier and Geyser both were competent to stand trial. Health experts testified that Geyser suffers from schizophrenia, but has refused to take medication. Weier was diagnosed with a delusional disorder that made her believe in Slenderman.

According to prosecutors, the girls lured the classmate into the woods and stabbed her 19 times to impress Slenderman, a fictional supernatural Internet character depicted in stories as stalking and tormenting humans, especially children. The stabbing occurred after a sleepover and had been planned for months.

The victim was found crawling out of the woods by a bicyclist. She spent six days in the hospital and returned to school last fall.

(Reporting by Brendan O'Brien in Milwaukee, additional reporting by Timothy Mclaughlin in Chicago; editing by Toby Chopra, Jeffrey Benkoe and Diane Craft)