(CNN) -- The teenager accused of gunning down his parents and three siblings in their New Mexico home had hoped to go on a killing spree and die in a shootout with police, investigators said Tuesday.
Nehemiah Griego, 15, 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.
Griego told investigators he hoped to end his spree "at a Walmart, with mass destruction," Bernalillo County Sheriff Dan Houston told reporters.
Griego told investigators he was "frustrated with his mother," but "he did not articulate that to our investigators any further or give any explanation of that at all," Houston said.
"This is beyond any human reasoning or understanding at this time," he said.
The teen had been 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, according to CNN affiliate KOAT reported. The station said Griego's case will go to a grand jury in 10 days.
Houston said the arrest came after Griego went to the family's church with his girlfriend, to whom he had sent a picture of his slain mother, and a staff member called deputies.
The youth's father, Greg Griego, was a former pastor at an Albuquerque church and a chaplain to the city's fire department who also worked with prisoners. He's believed to have been the last of the victims, gunned down about 5 a.m. Saturday when he came home from working the graveyard shift at a homeless shelter, Houston said.
The killings had begun about four hours earlier, when Nehemiah Griego shot and killed his mother, Sarah Griego, while she slept. The shot from the .22-caliber rifle woke his 9-year-old brother Zephania, who had been asleep next to her, an arrest affidavit recounted.
When the teen told Zephania that their mother was dead, "Nehemiah stated his brother did not believe him," the affidavit states. "So Nehemiah picked up his mother's head to show his brother her bloody face. Nehemiah stated his brother became upset, so he shot his brother in the head with the same rifle used to kill his mother."
After that, "Nehemiah stated he lost his sense of conscience" and went on to kill his sisters, 5-year-old Jael and 2-year-old Angelina, police said.
After the slayings, Griego "spent the majority of the day with his girlfriend," whose conduct remains a focus of the investigation, Houston said.
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. The Griegos had five other grown children, none of whom lived at home, Covington said.
Police said in a statement the weapons appear to have been purchased legally.