(CNN) -- The chilling acts the 15-year-old boy is accused of defy imagination:
Pumping his mother, brother and two younger sisters with bullets.
Gunning down his dad when he returned home.
Texting a picture of his lifeless mother to his 12-year-old girlfriend.
Plotting to kill strangers outside a supermarket.
But, family members say, Nehemiah Griego is no monster. They can't fathom what could have gone so terribly wrong.
"Whether it was a mental breakdown or some deeper undiagnosed psychological issue, we can't be sure yet," his uncle, former New Mexico state Sen. Eric Griego, said.
"What we do know is that none of us, even in our wildest nightmare, could have imagined that he could do something like this."
The family statement, sent to CNN affiliate KOAT, describes the boy as a gifted athlete and musician who was active in youth services at church.
It said the teen accompanied his father on mission trips to Mexico and wanted to continue a long family history of military service by becoming a soldier.
It's a far different picture of the teen than the one authorities paint.
Sheriff: Teen planned shootings for at least a week
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 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 a Walmart "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 tragedy unfolds
The tragedy unfolded 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 used to kill his mother."
After that, police said, "Nehemiah stated he lost his sense of conscience."
The boy went on to kill his sisters, 5-year-old Jael and 2-year-old Angelina, 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.
'Beyond any human reasoning'
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.
The Griegos had five other, grown children, none of whom lived at home, Covington said.
After the slayings, Griego "spent the majority of the day with his girlfriend," Houston said.
He even sent his girlfriend a picture of his slain mother, Houston said.
Authorities arrested the teen after he attended his family's church with his girlfriend, and a staff member called deputies.
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, KOAT reported.
The affiliate said Griego's case will go to a grand jury within 10 days.
"This is beyond any human reasoning or understanding at this time," the sheriff said.
-- CNN's Leslie Holland contributed to this report.
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