Last Updated Dec 21, 2017 5:46 PM EST
WAUKESHA, Wis. -- One of two Wisconsin girls whonamed was sentenced Thursday to 25 years in a mental hospital, the maximum punishment possible, with credit for time served in juvenile detention.
Waukesha County Circuit Judge Michael Bohren on Thursday sentenced Weier to 25 years in a psychiatric institution, retroactive to the date of the 2014 crime.
Anissa Weier, 16, pleaded guilty in August to being a party to attempted second-degree intentional homicide, but she claimed she wasn't responsible for her actions because she was mentally ill. In September, a jury agreed.
Weier and Morgan Geyser lured Payton Leutner into a wooded park in Waukesha, a Milwaukee suburb, in 2014. Geyser stabbed Leutner 19 times while Weier urged her on, according to investigators. Leutner survived after she crawled out of the woods to a path where a passing bicyclist found her.
Both Weier and Geyser told detectives they felt they had to kill Leutner to become Slender Man's "proxies," or servants, and protect their families from him. All three girls were 12 years old at the time.
Weier spoke to the court Thursday and apologized for her actions.
"I want everyone involved to know I deeply regret everything that happened that day," Weier said. "I know nothing I say is going to make this right, nothing I say is going to fix what I broke."
Weier said she holds herself accountable and said, "I'll do whatever I have to do to make sure this doesn't happen again."
Leutner's mother, Stacie Leutner, sent the judge a letter this week in which she wrote that the trauma of the attack "has defined our lives" and that her daughter still fears for her life. For months, Payton slept with scissors under her pillow for protection, and she still keeps her bedroom windows closed and locked.
"She will struggle with the events of that day and physical and emotional scars it left for the rest of her life," her mother wrote.
In her letter, Leutner did not ask the judge to sentence Weier to a specific length of time in the mental hospital, but she said her daughter wouldn't feel safe if either of her attackers is released back into the community unsupervised.
"Payton has a lifetime of healing ahead of her and she deserves to be allowed to heal in an environment where she feels safe," her mother wrote.
In a deal with prosecutors, Geyser, who did the stabbing, pleaded guilty to attempted first-degree intentional homicide with the agreement that she isn't criminally responsible and shouldn't go to prison.
Prosecutors said Thursday they agreed that Weier should be committed to a mental health facility rather than spend time in prison, but they asked for a term of 25 years from the date of the crime, when Weier will be 37. The term "pales in comparison" to the lifetime of suffering the victim will face, said Assistant District Attorney Kevin Osborne.
Weier's defense attorney Maura McMahon argued that she was very young at the time of the crime and asked the judge that she be committed until she is 25. Weier's family spoke via a video played to the court, saying doctors found that her risk of offending again is "nil."
"There were times when she said, 'I don't know why I would have done this,'" her grandmother Melody Weier said in the video. "She couldn't figure it out, and that would bother her a lot."
Weier's father spoke in the video and asked for forgiveness from the Leutner family.
"She's grown mentally and physically," her father William Weier said. "She knows what she did was wrong."
Bohren read from the victim's mother's statement in court and said it outlined the devastating impact of the crime, which created "longstanding, deep wounds I doubt will ever heal."
Bohren said Weier appears to be mature in court and is remorseful for her actions, but said he took into account that the crime was planned and that the intent was to kill the victim. He also noted that doctors have reported as recently as July that she still shows some of the delusional characteristics that factored into the crime, including that she believed an "evil spirit" had escaped a homemade ouija board she was playing with and was "pushing down on her bed."
Bohren said he factored in the safety of the community into his sentence and said it appears Weier still has mental health issues.
"As much as we can say 'I will never do it again' – and I think it's sincere – we don't know," Bohren said.
Slender Man started with an online post in 2009, as a mysterious specter whose image people edit into everyday scenes of children at play. He is typically depicted as a spidery figure in a black suit with a featureless white face. He was regarded by his devotees as alternately a sinister force and an avenging angel.