A Wisconsin judge ruled Monday that two 13-year-old girls accused of stabbing a classmate to please the online horror character Slender Man will stay in adult court, where they could face a sentence of decades in prison.

Waukesha County Circuit Judge Michael Bohren said he was worried that the girls would stop receiving mental health treatment and be released into the community with no supervision when they exited the juvenile system at age 18. Keeping them in the adult system would protect people longer, the judge said.

"The real issue is what happens in a few years," Bohren said. "They've committed an offense that was ... frankly vicious."

The two girls, both wearing dresses and in shackles, said nothing during the 45-minute proceeding. One of them bounced in her chair until a bailiff whispered to her. She spent the rest of the hearing hunched over and glancing at the ceiling. The girls' attorneys also said nothing to the judge.

One of their attorneys, Maura McMahon, told reporters outside the courtroom that she was "of course" disappointed and her client didn't understand what had happened. She said she planned to discuss an appeal with the girl and the attorney then left. The other defense attorneys and prosecutors departed without speaking to reporters.

The girls are both from Waukesha, a conservative Milwaukee suburb. They each face a charge of attempted first-degree intentional homicide in connection with the May 2014 attack on their classmate, Payton Leutner. According to a criminal complaint, the girls plotted for months before they lured Payton into some woods after a sleepover and attacked her with a knife. Payton was stabbed 19 times but survived.

The girls told investigators they hoped killing Payton would please Slender Man, a character they had read about in online horror stories. The tales describe Slender Man as an unnaturally thin, faceless creature who preys on children.

Police captured the girls on the outskirts of the city that same day. They told investigators they planned to walk 300 miles to the Nicolet National Forest, where they hoped to live as Slender Man's servants in his mansion.

All three girls were 12 years old at the time. Anyone 10 or older charged with first-degree attempted homicide is automatically considered an adult under Wisconsin law.

The girls could face up to 65 years in the state prison system if they're convicted as adults. The juvenile system, in contrast, is geared more toward rehabilitation than punishment. The girls could be held as juveniles for only five years.

Their attorneys had argued they belong in juvenile court because their brains aren't fully developed, they suffer from mental illness and they won't get the treatment they need in the adult system. They also asked Bohren to find the state law forcing 10-year-olds into adult court unconstitutional because it leads to cruel and unusual punishment.

Bohren refused to find the statutes unconstitutional on Thursday, writing that juveniles aren't as culpable for their actions as adults but that doesn't exempt them from adult sentences.

He rejected the rest of the defense teams' arguments on Monday. He acknowledged what he called the girls' delusions and mental illness, but said if he moved them into the juvenile system they'd be free and clear at age 18 with no oversight and no more treatment. If they are convicted as adults, they'd eventually be released on extended supervision and treatment would continue.

He said keeping the girls in the adult system would protect the community, noting again that their alleged crime was no accident.

"This was an effort to kill someone," the judge said. "This was premeditated murder."

The judge set the girls' arraignment, the point in the criminal justice system when defendants enter pleas, for Aug. 21.

The Associated Press isn't naming the girls because an appeals court could still move their cases to juvenile court, where proceedings are closed to the public.