LONG BEACH, Calif. >> Cecilia Villanueva sat helplessly in the backseat and watched her 10-year-old daughter turn blue as her husband raced them to the hospital in a panic.

Earlier, the fifth-grader, Joanna Ramos, had come home from school vomiting and complaining of a headache after a fight with another girl.

Before she passed out on the family couch, she told her mother an 11-year-old girl had punched her in the head.

"I could see her lips turning purple and I got so scared. I tried to do CPR," her mother said Tuesday, choking back tears. "I tried my best, but when we got to the hospital they said her heart was stopped. They tried, they tried so hard."

Joanna was pronounced dead on Friday night after undergoing emergency surgery for a blood clot on her brain, her 17-year-old sister, Vanessa Urbina, said.

The Los Angeles County coroner’s office labeled the case a homicide and said Joanna died of blunt force trauma to the head. Police said they have made no arrests and were conducting an investigation that will be presented to prosecutors when it’s completed.

The girl’s family and friends are stunned and struggling to understand how a schoolyard fight over a boy could end in death for a bubbly girl who loved to dance and sing, religiously followed soap operas on TV, and had a penchant for curling her long, dark hair. Joanna would have turned 11 on March 12.

Villanueva said she is certain her daughter didn’t tell her the whole story, and she is wary of believing the rumors that have been circulating among Joanna’s classmates.

"I told the doctor what happened and he said, ‘One punch is not enough, the way that she is right now,’" Villanueva said. "My daughter told me one punch, only, just one. And the doctor said, ‘Hmmm, I don’t think so. One wouldn’t cause too much damage.’"

Police have said the fight in an alley after school on Friday lasted less than a minute, involved no weapons and no one fell to the ground.

Villanueva, 41, said that before Joanna lost consciousness, she told her mother the other girl was her "enemy" but offered no further explanation.

"I said, what happened, and she said, ‘a girl punched my head,’ and I said why, and she said, ‘I don’t know Mom. We are enemies,’" Villanueva recalled. "I asked her, you don’t have any enemies. Why, Joanna? She told me, ‘I don’t want to talk, I’m tired and I want to go to sleep.’"

"After that she didn’t say anything no more," the mother said.

While the circumstances of Joanna’s death are tragic and extremely unusual, medical experts said a blow in just the right spot can often prove fatal.

"This is rare, in that I’ve never seen it in a female, certainly not in a female adolescent," said Dr. Keith Black, a neurosurgeon at Cedars-Sinai Medical Center.

Black, who was not involved in Joanna’s medical care, sees such injuries all the time among older patients, and said a blow to the head from one young girl to another could "absolutely" be sufficient to cause enough trauma to lead to death.

Punches to the head can often lead to delayed bleeding if a vein is torn, and that can lead to a clot when blood collects on the surface of the brain.

In a Tuesday column printed in The Press-Telegram of Long Beach, the trauma surgeon who treated Joanna in the emergency room said she suffered bleeding inside her skull and arrived at the hospital in grave condition. She went into cardiac arrest four times before she was finally pronounced dead, he said.

"Her eyes were ‘fixed and dilated,’ the worst sign possible. To a layperson, they look like the lifeless eyes of a little child’s doll," Dr. Mauricio Heilbron Jr. of St. Mary’s Medical Center wrote in the column.

The death has rattled 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.

School officials believe the fight occurred near the school in a 15-minute window between the time school let out and the start of Joanna’s after-school program at 2:30 p.m., said Chris Eftychiou, a spokesman for the Long Beach Unified School District.

Joanna didn’t have any visible injuries or show any signs of distress for about an hour, but she eventually told staff she felt unwell and was picked up by a relative, he said.

Symptoms — such as headache, nausea, lethargy — may not set in for hours and people can mistakenly think that they’re fine, Black said.

Typically, he said, the hit to the head would have to be fairly significant to cause a blood clot and often involves the head hitting walls or the ground, but a punch is enough.

Fights involving young children, including girls, are increasing nationally, in part because of the wired world children now live in, said Travis Brown, a national expert on bullying and school violence.

Children used to have a disagreement at school and would have a night or a weekend to cool down, but social media and text messaging mean students can continue their dispute 24 hours a day.

"There was a time when a kid had a way to escape the things at school, but now there’s no escape," Brown said. "That stuff just escalates to a point where it gets out of hand. This is an everyday occurrence."

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Associated Press writers Robert Jablon, Alicia Chang and Andrew Dalton in Los Angeles contributed to this report.