A 12-year-old California girl who died in February after allegedly being struck in the head during what her family said was a bullying incident had a pre-existing medical condition that led to her death, according to the medical examiner.

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The Los Angeles County Medical Examiner ruled that Khimberly Zavaleta died of natural causes, listing her cause of death as “spontaneously ruptured cerebellar arteriovenous malformation.”

Khimberly’s family said that she was hit in the head with a metal water bottle thrown by a fellow student at Reseda High School on Feb. 17, while she was trying to protect her sister during an alleged bullying incident. She suffered a brain injury, spent several days in a coma, and underwent surgery before dying at the hospital.

In a press release issued Tuesday, the medical examiner’s office said Khimberly was initially brought to the hospital on Feb. 17 complaining of a headache but was discharged. Four days later, she was rushed “in distress” and diagnosed with arteriovenous malformation (AVM).

AVM is a rare, congenital tangle of blood vessels with no capillaries between them, according to the Cleveland Clinic. It often goes undetected until it bleeds; when a brain AVM ruptures, it can cause a stroke and severe brain damage. The medical examiner’s office said Khimberly received “significant medical treatment” but died on Feb. 25.

“Arteriovenous malformations (AVM) are an assembly of fragile, tangled, high-pressure blood vessels that are prone to spontaneously rupturing, especially when located in the region of the brain as discovered in Khimberly,” Chief Medical Examiner Dr. Odey Ukpo said in a statement. “Catastrophic bleeding due to a rupture develops quickly — within seconds to minutes — and is immediately life-threatening. Unfortunately, as in Khimberly’s case, spontaneous rupture is a common initial presentation of an AVM.”

The Los Angeles Police Department had launched a homicide investigation after Khimberly’s death, and a child was arrested in April, according to NBC Los Angeles. The department did not immediately respond to a request for comment Wednesday on the status of the case.

An attorney representing the family called the medical examiner’s findings inadequate on Wednesday. “Before this incident, Khimberly was a healthy, vibrant 12-year-old girl with no symptoms, no medical crisis, and no indication that her AVM posed any danger to her life,” Robert Glassman said in a statement. “Then she was struck in the head at school with an aluminum water bottle, complained of serious head pain, and within days suffered catastrophic brain bleeding that took her life. No press release by the medical examiner changes those facts.”

The family filed a civil lawsuit against the Los Angeles Unified School District in March, alleging negligence, inaction, and a failure to protect and properly respond to a child in distress. The case remains pending.

“If Khimberly had an underlying condition that made her more vulnerable to injury, that does not excuse the conduct that led to her death. It makes the need to protect her even greater,” Glassman said. “Khimberly was alive before this happened. She is gone now. Plain and simple. We will continue fighting for accountability for Khimberly and her family.”