Blunt force trauma to the head killed Joanna Ramos, who collapsed at home after Friday’s fight, coroner’s Lt. Fred Corral said. He did not immediately have details about her injuries.

The girl’s older sister said Joanna died after surgery for a blood clot on the brain after the fight in an alley with an 11-year-old girl. Joanna had started vomiting and complained of a headache and was unconscious by the time she arrived at the emergency room, said Vanessa Urbina, 17.

“After surgery the doctor said she was still alive, and then a few minutes later he comes back and tells us that her heart stopped and they couldn’t bring her back,” Urbina said, crying, as she sat on the steps of Willard Elementary School near a memorial of flowers and balloons.

Joanna was pronounced dead about six hours after she and the 11-year-old had their planned fight.

No arrests have been made. The Long Beach Police Department said in a statement Monday that it was continuing to conduct a “very intense and detailed investigation.”

“The District Attorney’s Office will ultimately determine whether charges may be filed in this case,” the statement said.

Doctors said it’s not uncommon for someone to experience delayed bleeding after a head injury.

Joanna most likely suffered a blood clot that developed when blood from a torn vein collected on the surface of the brain, said neurosurgeon Dr. Keith Black of Cedars-Sinai Medical Center, who was not involved in her medical care.

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

“Just because you’re OK immediately after a head injury, you still need to be very concerned” about pressure buildup in the brain that can be life-threatening, Black said.

Typically, the hit to the head would have to be fairly significant to cause a blood clot, he said.

Friday’s fight lasted less than a minute, police said. It didn’t involve weapons, and no one was knocked to the ground.

“They took off their backpacks, and they put their hair in a bun, and then that’s when they said ‘Go’ and that’s when they started hitting each other,” Joanna’s friend and classmate Maggie Martinez, who watched the fight, told KNBC.

Martinez and other friends said they tried to stop the fight but were held back by boys who were watching and wanted it to continue.