Jury selection was to begin Monday in the trial of one of two US teens accused of stabbing a friend to appease a fictitious internet character.

Anissa Weier and Morgan Geyser - now 15, but 12 years old at the time of the 2014 crime - were charged with attempted first-degree murder for the brutal attack blamed on their belief in Slender Man - an online horror villain.

Weier pleaded guilty in August to a reduced attempted second-degree homicide charge in a Wisconsin court. Her trial will determine her mental health and whether she is Pto be sentenced to prison or a hospital.

The two girls are accused of stabbing a friend, Payton Leutner, 19 times at a park. A bicyclist found Leutner, who survived the attack, and helped her get to a hospital.

The incident inspired the 2016 HBO documentary "Beware the Slenderman."

The girls told police they were trying to please the fictitious character, and believed he would hurt their families if they did not stab their friend.

"Morgan jumped on top of Bella and started stabbing her repeatedly, and that's when I turned around because I couldn't stand to see that," Weier told police in an interrogation video made public, using a nickname for the victim.

Geyser, during her own police interview, claimed it was Weier who jumped on Leutner and "held her to the floor."

"I think Anissa stabbed her first, and then I continued."

Both teens have asserted insanity defenses. Geyser is to stand trial later this year.

Under the terms of Weier's guilty plea, prosecutors would recommend 10 years in prison if the jury rejects her insanity defense.

But she would be committed to a mental hospital for at least three years if the jury finds her impaired by mental illness.

Psychologists testified in pre-trial hearings that Weier suffered from depression and a delusional disorder that made it difficult to tell reality from fantasy, according to US media.

Geyser was being treated at a mental hospital for schizophrenia.

The trial is scheduled to last until September 21.

Source: Khaleej Times