The 2-and-a-half-year-old girl, Erica Johnson, lived in the house with her grandmother, a 17-year-old male and a 21-year-old man.
The victim's 19-year-old mother is in custody, arrested last week in an unrelated incident. The little girl's father is in custody in another state.
The victim's grandmother said the 21-year-old suspect, her daughter's boyfriend, started banging on her bedroom door about 11 p.m. She said he told her the baby wasn't breathing. They dialled 911.
"At about 11:15 last night, which would have been Wednesday night, a 2-and-a-half-year-old girl was transported to Centinela Hospital in full cardiac arrest," said L.A. County Sheriff's Lt. Pat Nelson. "The doctors were unable to revive her. And doing their attempts to do so, noticed that there was some traumatic injuries that may have contributed to her death, in the doctors' opinions."
The child's death was ruled a homicide Thursday afternoon. The 21-year-old was arrested and booked for the girl's murder.
An autopsy is pending to determine the cause of death, according to the L.A. County Coroner's Office.