ALBUQUERQUE, N.M. – The New Mexico teenager accused of gunning down five family members over the weekend ambushed his father as he returned home from an overnight shift at a rescue mission, then reloaded his rifles and planned to go to a Walmart and randomly shoot people, authorities said Tuesday.

Instead, 15-year-old Nehemiah Griego texted a picture of his dead mother to his 12-year-old girlfriend, then spent much of Saturday with the girl and her family, authorities said. That evening, the teen went to the church where his father had been a pastor, and Griego eventually confessed to killing his parents and three younger siblings.

"The motive, as articulated by the suspect, was purely that he was frustrated with his mother," Bernalillo County Sheriff Dan Houston said. "He did not give any further explanation."