"The motive was purely that he was frustrated with his mother. He could not articulate to our investigators any farther," Houston said. "In the time our investigators spent with him, it was a very casual (statement), he was just frustrated with how things were, and would not even articulate any further details of that frustration."
"It's horrific," Houston added.
A police report from the incident shows that Griego admitted to having "homicidal and suicidal thoughts" in the time leading up to the incident.
Griego reportedly gushed to police about his love for violent video games during the interrogation, Houston said. He told police he loved to play Modern Warfare and Grand Theft Auto.
"The suspect was involved heavily in games, violent games, it's what he was into," Houston said. "He was quite excited as he discussed this with our investigators."
Houston said that Griego had occasionally lost touch with his family and then reconnected with them multiple times in his life. He told investigators that his father had taught him how to shoot the weapons and the pair had practiced shooting them together.
Griego has five older siblings who were not living at the home at the time of the shooting and were unharmed.
He is facing murder and child abuse charges and will be tried as an adult, according to police.