Long Beach police are awaiting autopsy results to determine exactly how the girl died and whether the fight she had earlier with a classmate contributed to her death.
The victim was a 5th grader at Willard Elementary School, identified by friends and family members as Joanna Ramos.
Outside the school on Saturday, loved ones left balloons, candles and flowers, creating a makeshift memorial to pay tribute to Ramos.
Long Beach police say Ramos died after getting into a fight on Friday after school got out. Students say the fight happened at an alley near the school, not on campus.
Long Beach police Deputy Chief Robert Luna said the two girls and seven onlookers went to an alley, where they had planned to fight.
Friends said they were fighting over a boy.
Investigators said no weapons were involved and no one was knocked to the ground.
Friends say Ramos and another fifth grader had an ongoing feud. The pair allegedly fought on Thursday and again on Friday.
One friend said she complained of not feeling well in the afternoon during an after-school program they both attended.
"She was crying and her head was hurting," said Ramos' classmate Justin Robert King Jr. He added that Ramos told the supervisor of the after-school program about pain in her head.
Police said Ramos was unconscious and not breathing when family members took her to the hospital at about 5:50 p.m. She was rushed into surgery but was pronounced dead at 8:50 p.m.
If you any information that could help police, you're urged to call Long Beach detectives at (562) 570-7244.
CORRECTION: In earlier reports we said that Joanna Ramos was 11 years old. She was 10 years old at the time of her death.