PHOENIX -- A Valley man is behind bars, accused of beating a 3-year-old boy to death.

Police arrested 20-year-old Robert Celaya after his girlfriend's son died Saturday at St. Joseph's Hospital.

Celaya faces first-degree murder and child abuse charges in the death of Anthony Hernandez. He is being held on $81,000 bond.

Officials said doctors called police after finding severe trauma to the child's abdomen.

Court records indicated the medical examiner ruled the death a homicide. The injuries included multiple bruising to the left side of the rib area and a severed intestine, according to the records.

"I don't understand how people can do that to little children that small," Clarissa Murrieta, a neighbor, told CBS 5 News. "They're innocent children. They don't deserve to be treated like that."

Neighbors told CBS 5 News they saw investigators at the home all day Saturday collecting evidence and taking pictures.