A New Mexico teenager pleaded guilty in the shooting deaths of his parents and three young siblings on Friday, nearly three years after authorities say he opened fire at his family's rural home south of Albuquerque.

Nehemiah Griego entered the guilty pleas in state children's court to two counts of second-degree murder in the deaths of his parents and three counts of child abuse resulting in death in the slayings of his three younger siblings — Zephaniah, Jael and Angelina. He was 15 at the time of the January 2013 shooting.

The brief hearing ended more than two and a half years of litigation as Griego received extensive treament from the state, said Griego's attorney Jeffrey Buckels. A weeklong hearing will be held in mid-January to decide whether Griego, who is now 18, will be sentenced as a juvenile or an adult.

"The parties believe that this agreement is the best resolution for Mr. Griego and the State, as well as for the victim's family, which was fully apprised of the agreement in advance of (the) plea hearing," Buckels said in a statement.

Griego's sentencing terms could range from probation to three life sentences plus 30 years if he is sentenced as an adult, Buckels said. If he is sentenced as a juvenile, he would theoretically remain in the custody of the state Children, Youth and Families Department until he is 21, but that will be up to the courts to determine after hearing from investigators, witnesses and psychology experts in January.