OMAHA, Neb. (AP) — Douglas County prosecutors say an autopsy shows a boy thrown into the Elkhorn River on the eve of his fifth birthday drowned.

The autopsy says Josue Ramirez-Marinero, whose tiny body was pulled from the river Monday, showed no signs of injury or trauma.

Police say Josue’s 25-year-old half brother, Roberto Martinez-Marinero, threw the boy into the river on May 5 because the boy had witnessed his mother’s killing at the hands of Martinez-Marinero. Police reports say he would have turned 5 on May 6.

Police say Martinez-Merinero also told investigators he had killed his mother, Jesus Ismenia Marinero, on May 5. Her body was found in a southeastern Omaha ditch.

Martinez-Marinero is charged with two counts of first-degree murder. His girlfriend, 24-year-old Gabriela Guevara, is charged with being an accessory.