Nehemiah Griego, 15, of New Mexico, is accused of gunning down his mother, younger brother, two younger sisters and father in their home in Albuquerque on Saturday, January 19.
Bernalillo County Sheriff Dan Houston said Griego was "involved heavily" with violent video games and that he "was quite excited as he got the opportunity to discuss that with our investigators."
He said that the boy planned the shootings for at least a week. That he told investigators he was frustrated with his mother. That he contemplated killing his girlfriend's parents. That he hoped to continue his killing spree at Wal-Mart "with mass destruction" and die in a shootout with police.
The teen's demeanor, as he shared all this, was "stern, very unemotional," Houston said.
Now, Griego will stand trial as an adult in the weekend killings, prosecutors announced.
He was arrested Saturday night, after deputies found the bodies of his mother, father, brother and two of his sisters in their home on the outskirts of Albuquerque.
The murders took place early Saturday morning, when Griego carried a .22-caliber rifle to where his mother and 9-year-old brother slept side by side, an arrest affidavit said. The teen fatally shot his mother, waking his younger brother Zephania in the process. When Griego told Zephania that their mother was dead, the younger boy did not believe him, the document stated.
"So Nehemiah picked up his mother's head to show his brother her bloody face," the affidavit said. "Nehemiah stated his brother became upset, so he shot his brother in the head with the same rifle he used to kill his mother."
After that, police said, "Nehemiah stated he lost his sense of conscience." He went on to kill his sisters, Jael, 5, and Angelina, 2, before his father Greg came home from a volunteer graveyard shift at a homeless shelter.
Authorities believe the father, a former pastor at an Albuquerque church and a chaplain to the city's fire department, was the last to be gunned down in the home.
Most of the victims were shot with the .22, but a semiautomatic AR-15 rifle was also used in the killings, sheriff's Lt. Sid Covington said. Authorities said the weapons appear to have been purchased legally.
Family members described Nehemiah as a gifted athlete and musician who was active in youth services at church. They said he had accompanied his father on mission trips to Mexico and wanted to continue a long family history of military service by becoming a soldier.
The Griegos also had five grown children, none of whom lived at home, Covington said.
After the slayings, Griego spent the majority of the day with his 12-year-old girlfriend, Houston said. He even sent his girlfriend a picture of his slain mother.
Authorities arrested the teen after he attended his family's church with his girlfriend, and a staff member called deputies.
Pastor Skip Heitzig said father Greg Griego was also a pastor for a number of years who did prison ministry at the Metropolitan Detention Center and helped to rehabilitate convicts.
Griego was scheduled to appear in court Tuesday to face two counts of murder and three counts of child abuse resulting in death, but the hearing was waived. Griego's case will go to a grand jury within 10 days (CNN.com).