Fifteen-year-old Alyssa Bustamante aged considerably Wednesday when a Missouri judge ruled that she should be tried as an adult for allegedly strangling a 9-year-old neighbor a week after digging a grave for the girl.
The judge also entered a not guilty plea on her behalf after she was indicted on charges of first-degree murder. She is being held without bond.
The teenager had told state police that she wanted to know what the experience was like, a highway patrol officer said.
"Ultimately, she stated she wanted to know what it felt like," Missouri State Highway Patrol Sgt. David Rice testified in court Wednesday.
She also allegedly cut the girl's throat and stabbed her.
"She committed the murder after deliberation, which means cool deliberation or cool reflection on the matter for any length of time," Cole County prosecutor Mark Richardson told the court Wednesday.
Elizabeth Olten's body was found in a wooded area near her home in St. Martins Oct. 23 after the suspect led police to it, after which she was arrested, authorities said. The girl had vanished Oct. 21, but it wasn't long after that police found what Cole Country Sheriff Greg White called "written evidence" that led them to Bustamante.
If convicted, Bustamante could be sentenced to life in prison.
The suspect did not have a prior criminal record, police said
But there were signs of a troubled past.
On the teen's YouTube page, a video appears to show the suspect with her brothers purposefully shocking themselves on an electrified fence.
On what appears to be her Twitter page, her messages speak of "addiction" and "terrors."
One Tweet: "& all I want in life is a reason for all this pain."
A grim portrait of Bustamante emerged earlier based on writings on the Internet, including a YouTube profile in which "killing people" is listed under the girl's hobbies.
In court, officials said the teenager had attempted suicide in 2007 and has been taking an antidepressant ever since.
"If one of the priorities or focus of her life everyday is pain, then it would be natural to pull people into it, to inflict pain upon them," said Brad Garrett, a former FBI profiler and ABC News consultant.
Kurt Valentine, Bustamante's attorney, said the teenager had recently tried to harm herself while in custody by cutting herself with her fingernails.
Valentine had urged officials to be cautious with their judgments of the suspect.
"I would ask that they wait, that they listen to the facts as they come out and not judge quickly," Valentine said. "Learn about this person, learn about this child. You're dealing with a child."
ABC News' Emily Friedman and Lee Ferran, as well as the Associated Press, contributed to this report.