The girl's injuries included a skull fracture, swelling of the brain and bleeding around the brain that resulted from “significant forcible head trauma,” a doctor who treated the girl testified at Rivera's preliminary hearing.
The boy was playing the game “Call of Duty: Black Ops,” authorities said. The warfare-based game is rated for players 17 and older.
Rivera told police his sister fell and started to cry, distracting him. The boy “confessed that when his man in the video game was killed, he became angry and frustrated,” and he picked up his sister and shook her, police said.
Rivera, who was 13 when his sister died, is being tried in juvenile court. He wore jeans and a T-shirt, and showed no emotion Tuesday.
In February, a judge ruled Rivera should be certified as a youthful offender after a case worker from the Office of Juvenile Affairs and a psychologist testified the boy can be rehabilitated despite a propensity for anger.
Rivera's parents, Carlos Beletzuy-Lopez and Linda Walleska Beletzuy, of Oklahoma City, who left Rivera with the girl and his 4-year-old brother, are awaiting trial on child neglect charges.
The teen's trial continues Wednesday.