"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 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 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.
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. Greg Griego also helped them re-enter society.
"The news stunned us all," Heitzig said of the shooting.
"We are doing what we can as a church body to minister to the remaining family members. Only the Lord Jesus Christ can heal this type of pain and heartache, and He is faithful to do so," he added in a statement.
His church, Calvary Albuquerque, is scheduled to hold a prayer vigil Wednesday at 7 p.m. for the Griego family.
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.