ALBUQUERQUE, N.M. (AP) - For at least a week, authorities say, 15-year-old Nehemiah Griego had been planning the attack.
After shooting his mother and three siblings in their beds, he ambushed his father as the pastor returned home from an overnight shift at a homeless shelter. Then the teen reloaded the family's rifles.
His plan was to randomly shoot people at a Wal-Mart on Saturday, which happened to be "Guns Across America" day, until he could be killed in a shootout with law enforcement, according to authorities.
He also contemplated killing his 12-year-old girlfriend's parents, Bernalillo County Sheriff Dan Houston said Tuesday.
And while Griego loaded guns and ammunition into the family's van, Houston said, it was unclear whether the teen did go to a Wal-Mart or how seriously he contemplated continuing his rampage on the same day that thousands of gun advocates gathered peacefully at state capitals around the country to rally against stricter limits on firearms.
The "Guns Across America" events were held after President Barack Obama unveiled a sweeping package of federal gun-control proposals.
What authorities know, the sheriff said, was that Griego texted a picture of his dead mother to his girlfriend, then spent much of Saturday with her and her family, authorities said. That evening, he went to the church where his father worked and confessed to killing his parents and three younger siblings, authorities said.
The sheriff said he didn't know if Griego's contact with his girlfriend avoided further bloodshed. But he said she apparently knew what had happened, and officials were investigating whether she should be charged with failing to report the crime.
"We know Nehemiah had been contemplating this for some time," Houston told reporters at a news conference. Griego apparently had told others of his plans, but whom and when were still under investigation, Houston said.
The motive, Houston said, "was purely that he was frustrated with his mother. He did not give any further explanation."
The teen waived his right to arraignment in adult criminal court Tuesday on charges of murder and child abuse resulting in death and was ordered held without bond. He was arrested Saturday at his family's home in a rural area southwest of Albuquerque.
The sheriff's office identified the dead as Greg Griego, 51, his wife, Sarah Griego, 40, and three of their children: a 9-year-old boy, Zephania Griego, and daughters Jael Griego, 5, and Angelina Griego, 2.