Under the terms of the plea deal, Anissa Weier could be sentenced to 10 years in prison for stabbing her classmate
Girls Stabbing Plot
Credit: Michael Sears/Milwaukee Journal-Sentinel/AP

One of the two Wisconsin girls accused of stabbing a classmate 19 times to please the fictional online character “Slenderman” pleaded guilty Monday to a downgraded charge, but will still go to trial next month to determine her mental fitness, PEOPLE confirms.

Anissa Weier, 15, pleaded guilty to attempted second-degree homicide as a party to a crime with use of a deadly weapon, court officials confirm. She was initially charged with attempted first-degree intentional homicide in the 2014 attack on Payton Leutner, then 12, in Waukesha, Wisconsin.

Under the terms of the plea deal, Weier could be sentenced to 10 years in prison if, at the conclusion of next month’s trial, a judge finds that she was mentally competent and therefore legally responsible for the attempted murder.

If the judge determines Weier was not mentally competent, she will likely be committed to a mental hospital for three years, PEOPLE learns.

Prosecutors allege that Weier and Morgan Geyser, both 12 on May 31, 2014, lured Payton — also 12 — to a local park where she was found alone and stabbed 19 times. Payton was left to die, but managed to crawl to a bike path where a passerby found her and called 911, according to investigators.

The girls were charged as adults with attempted first-degree intentional homicide. They initially pleaded not guilty by reason of mental illness or defect.

Weier and Geyser, also 15, allegedly told police they committed the stabbing in the name of the “Slenderman.” After the stabbing, the girls said they planned to walk hundreds of miles to meet Slenderman.

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Geyser’s trial will commence in October, officials tell PEOPLE. According to her parents, Geyser has been diagnosed with early-onset schizophrenia. Weier has no known history of mental illness.

Payton’s family issued a statement to PEOPLE thanking prosecutors for their hard work.

“It has been more than three years since our daughter was brutally attacked by two classmates who premeditatedly and meticulously planned their assault in an attempt to kill our daughter,” the statement reads. “These three years have been very difficult both physically and emotionally for Payton and our family. Paramount in our decision to accept today’s plea agreement is that it provides closure without having to have Payton testify and be forced to relive this horrific incident.

“Though perhaps not to the extent in which we would hope these assailants be punished, we are forced to work within the confines of current law,” the statement continues. “Ultimately, our decision is what is best for our beautiful and amazingly brave daughter, Payton.”

PEOPLE was unable to reach Weier’s attorney for comment.

• With reporting by ELAINE ARADILLAS