A MISSOURI teenager who told authorities she wanted to know what it felt like to kill has pleaded guilty to murder.

She told a judge she strangled her nine-year-old neighbour with her hands, stabbed her in the chest and cut her throat with a knife.

Alyssa Bustamante pleaded guilty to second-degree murder and armed criminal action in the October 21, 2009, killing of Elizabeth Olten in St Martins, a rural town just west of Jefferson City. The plea avoids a trial that had been scheduled to start later this month.

At Tuesday's hearing, Bustamante looked down, her long brown hair covering her eyes, as the judge read out the amended charges and asked her if she understood she was giving up her right to a trial. She replied, "yes''.

Cole County Circuit Judge Patricia Joyce then asked Bustamante to describe what she did.

"I strangled her and stabbed her in the chest,'' Bustamante said in a clear voice, looking straight at the judge.

"Did you cut her throat too?'' the judge asked.

"Yes," Bustamante responded.

Bustamante, who is to turn 18 on January 28, was 15 at the time of the slaying and lived just a few homes down the road from Elizabeth. She told the judge on Tuesday that she knew what she was doing when she strangled Elizabeth with her hands and used a knife to attack her.

Upon hearing that confession, Elizabeth's mother Patty Preiss - who was sitting in the courtroom just a few feet away - took a deep breath and dabbed her eyes with a tissue.

Preiss wore a purple shirt with her daughter's picture and the words Justice for Elizabeth. She left the courthouse without talking to the media.

Prosecutor Mark Richardson declined to comment after Tuesday's hearing, noting the case is ongoing.

The judge set a February 6 sentencing hearing. Bustamante would have faced life in prison without parole if convicted of first-degree murder. But by pleading guilty to a reduced murder charge, Bustamante could eventually being released. The punishment for second-degree murder ranges from 10-30 years in prison or life with the possibility of parole. The judge said the armed criminal action charge is punishable by three years up to life in prison.

Bustamante's lawyer, Charlie Moreland, said in an interview that Bustamante decided to plead guilty because "she wanted to take responsibility for it''.

Bustamante's grandmother, who had been her legal guardian, left the courthouse without commenting to reporters.

Juvenile justice officials testified at a November 2009 hearing that Bustamante had attempted to commit suicide at age 13 after receiving mental health treatment for depression and cutting herself.

At that 2009 hearing, prosecutors said Bustamante had plotted Elizabeth's death, even digging two holes to be used as graves, then attending school for about a week while waiting for the right time to kill.

Missouri State Highway Patrol Sergeant David Rice testified that the teenager confessed to the slaying and led authorities to Elizabeth's body. Rice said Bustamante told him "she wanted to know what it felt like'' to kill someone.