Apa collapsed at Mile 24, about two miles from the finish, CBS News reports. A bystander gave her CPR until medical staff from race organizer New York Road Runners took over, CBS says. A defibrillator was used and she was taken to a Mount Sinai hospital, where she died.

Her father, Steve Apa, said she had rhabdomyolysis, CBS and the Post report, citing his interview with The New York Times. “It’s unfathomable. Perfect health, perfect shape, 24 years old,” he said, the Post reports.

He had been texting her encouragement at every mile, the Post says. At Mile 18 she texted back “Ow,” and at Mile 19 she replied “Ow ow,” he said, according to the Post. Her mother was waiting near the finish line.

This was her first marathon, though she was a lifelong athlete who exercised regularly, her father said, NBC News reports.

Rhabdomyolysis happens when muscle cells break apart and spill a protein called myoglobin into the blood, along with potassium, CBS reports. Too much myoglobin can damage the kidneys, and high potassium can cause a deadly heart rhythm, CBS says.

About 26,000 people in the country are diagnosed with the condition each year, CBS reports, citing the Cleveland Clinic. It can follow very intense exercise, and the risk is higher in hot weather, when dehydrated or when sick, CBS says.

The New York City Office of the Chief Medical Examiner says no final determination of the cause of death has been made, CBS reports. New York Road Runners also did not confirm her cause of death. “We are deeply saddened by Rachel's passing following running in Sunday's race,” the group said.

What we know: Apa collapsed at Mile 24 and died, and her father says the cause was rhabdomyolysis.
What we don't: The official cause of death. The medical examiner has not made a final ruling.