The fight near Willard Elementary didn't appear to be especially violent, no weapons were used, and neither girl was knocked to the ground, police have said.
Joanna had a bloody nose when she returned to her after-school program, according to witnesses, and had to be picked up early by a relative because she didn't feel well.
By the time Joanna got home, she was complaining of a headache and vomiting.
Before she passed out on the family's couch, she told her mother an 11-year-old girl had punched her in the head but refused to say more, Villanueva previously told the AP.
The coroner's office labeled the case a homicide and said Joanna died of blunt force trauma to the head. The autopsy report remained under a security hold Wednesday, said Assistant Chief Coroner Ed Winter.
The death shook the school community at Willard Elementary, located in a working-class neighborhood just a few miles from a more affluent area of homes that front a bluff overlooking the Pacific Ocean.
Joanna, a bubbly girl who loved soap operas and curling her long, dark hair, would have turned 11 on March 12. In her honor, the family went to a local amusement park as she had requested for her birthday party, Urbina said.
"We're taking it day by day," she said. "I just miss her being here."