Sheriff: New Mexico Teen Had Been Planning Family Shooting

Also planned to go to local Wal-Mart and force shootout with police

For at least a week, 15-year-old Nehemiah Griego had been planning the attack, authorities said.

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.