The teenager accused of shooting dead his parents and three younger siblings at their New Mexico home has reportedly told authorities he was "annoyed" with his mother.
A probable cause statement said Nehemiah Griego had been having homicidal and suicidal thoughts before allegedly killing his family with an assault weapon.
The 15-year-old remains in custody on charges of murder and child abuse resulting in death following the shootings on Saturday night in the rural area southwest of Albuquerque.
A Bernalillo County sheriff's detective questioned the teenager and details of their conversation were spelled out in the statement.
Griego allegedly told the detective he took a .22 caliber rifle from his parents' cupboard and shot his mother in the head while his younger brother slept next to her.
The suspect told the detective his brother did not believe their mother was dead so he showed him her bloody face and then shot him, according to the statement.
He is accused of then shooting his two young sisters in their room before taking an AR-15 rifle from his parents' cupboard and waiting in a downstairs bathroom for his father to come home.
The statement said he shot his father multiple times after he passed the bathroom doorway.
The teen said he reloaded the guns so "he could drive to a populated area to murder more people", according to the statement.
It said his plan was to "shoot people at random and eventually be killed while exchanging gunfire with law enforcement".
Asked if he had told anyone else about murdering his family, Griego allegedly told the detective that he had taken a picture of his dead mother and sent it to his girlfriend.
Police in New Mexico have been collecting evidence from the home and Bernalillo County Sheriff Dan Houston said: "There's no other way to say it, except that we have a horrific crime scene down there that we are working on."
The victims were identified as Greg Griego, 51, his wife Sarah Griego, 40, and three of their children: a nine-year-old boy, Zephania Griego, and daughters Jael Griego, five, and Angelina Griego, two.
All appeared to have gunshot wounds to the head.
Greg Griego was a pastor and was also well-known throughout the law enforcement community for his work as a voluntary chaplain.
It comes after President Obama announced plans to ban assault weapons, following a massacre that left 28 dead in Newtown, Connecticut - also committed using an AR-15 rifle.