Roberto C. Martinez-Marinero, 25, reportedly admitted to authorities that he killed his mother, left his 11-month-old brother in a dumpster in La Vista and then threw his 5-year-old half-brother, Josue, over a guardrail and into the Elkhorn River.
Josue was alive when he was thrown into the river, Douglas County Attorney Don Kleine said.
The boy's body was discovered Monday morning about a mile south of the U.S. Highway 275/West Center Road bridge.
Kleine said two aggravating factors exist that would merit the death penalty under state law: There were multiple slayings, and the killing of Josue was done to eliminate a witness to the slaying of his mom, Jesus Ismenia Marinero.
Martinez-Marinero beat his mother in the face and head with a baseball bat then stabbed her multiple times, Kleine said.
Authorities also are trying to determine the exact role played by Martinez-Marinero's girlfriend, Gabriela Guevara. Guevara has been charged with five counts of being an accessory to the slayings of Ismenia Marinero and Josue.
She reportedly was with Martinez-Marinero at the killing of his mother at their apartment near 20th and Poppleton Avenue.