Prosecutors have charged both girls with attempted first-degree intentional homicide after the attack in May. They say the girls plotted for months to kill classmate Payton Leutner, luring her to a wooded park after a sleepover and stabbing her 19 times. All three girls were 12 at the time of the incident.

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Anissa Weier and Morgan Geyser have been charged as adults because Wisconsin law requires suspects who are at least 10 years old to be charged as adults in severe crimes.

Both their attorneys originally contested evaluations from state experts that found both girls were competent to stand trial.

According to ABC in the US, one of the girls Anissa Weier, 13, appeared in court and when asked by the judge whether she considered herself competent she said: “I consider myself incompetent, sir”.

Waukesha County Circuit Judge Michael Bohren held back-to-back hearings for both girls on Thursday to decide. He ruled that Weier was competent.

A second hearing for Geyser was later waived by her lawyers, who agreed with a doctor’s opinion that she was competent.

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Bohren’s decisions mean both cases will continue to preliminary hearings, where a judge will decide whether there’s enough evidence for trial. Attorneys have said they may still try to move their cases into juvenile court, where proceedings are secret.

The two girls were found walking toward a national forest where they said they believed Slender Man lived in a mansion. They told investigators they believed killing Leutner would curry favour with the figure.

After her attackers left, Leutner crawled through the woods to a sidewalk where a bicyclist found her and called for help.