Rachel Apa died Sunday before she could cross the finish line during her first marathon.

The 24-year-old woman experienced a medical episode near mile marker 24 in Central Park during the New York City Marathon on Sunday afternoon, her father confirmed to NBC News. The New York Police Department said that emergency medical workers took the woman to Mount Sinai Hospital in critical condition, where she died hours later.

Her father, Steve Apa, called the situation “unfathomable.”

Steve said his wife, Michelle, and two of his daughter’s friends spent Sunday traversing the five boroughs to follow Rachel, a former Division I field hockey player, as she ran her first marathon. He opted to follow along on the app from home in Lewisberry, Pennsylvania.

He said he spoke to Rachel before she started running and said she was “so excited about it.” During the race, they stayed in touch by exchanging short text messages.

The first text came around 11:30 a.m., when Rachel was at the 9-mile mark. “Hey, go,” she wrote, referring to a mantra the pair have shared for years and a nod to Steve’s days playing football at Lehigh University.

“I feel amazing,” she then wrote, chalking it up to her pre-race carb-loading, according to her dad. “I’ve never felt this good.”

Her dad responded with caution: “Great, don’t overdo it. First marathon. This is a learning run. Don’t overdo it.” He added that she’d be seeing her mom on the left side of the track in two blocks.

Steve said his wife saw their daughter fly by at the 14th mile before Rachel texted him, “Ugh,” a few miles later.

“You beat the 18,” he responded.

“Ow, ow,” Rachel said, spurring Steve to remind her to “go easy.”

At mile 20, Steve said he texted again. She responded with another “Ow, ow.”

He kept texting at mile 21, mile 22 and mile 23. At mile 23, Steve said: “All finish, not speed.”

Steve texted one more time at mile 24, around 1:30 p.m., before noticing that Rachel’s location on the marathon tracking app had stopped moving.

At that point, Steve said his wife and Rachel’s friends were waiting for her at mile 25 and also noticed her location had stopped moving. His wife then called him, and he told her to find their daughter.

He said he then texted his daughter to tell her that her mom was walking toward her.

She never replied.

Steve said his wife arrived near the mile 24 marker and started yelling Rachel’s name when she couldn’t find her. An EMS worker still on scene told Michelle that an ambulance had just taken her daughter to the hospital, and the worker drove Michelle to Mount Sinai in a second ambulance, he said.

Meanwhile, Steve started driving to New York.

New York Road Runners, which organizes the marathon, said in a statement that a bystander administered CPR before the marathon’s medical staff could. The organization said the medical staff then used a defibrillator on Rachel.

The doctors told the Apas that their daughter died of rhabdomyolysis, calling the diagnosis “extremely rare.” The Cleveland Clinic describes rhabdomyolysis as a serious condition that causes muscles to break down due to overexertion, allowing “toxic components of your muscle fibers” to enter the bloodstream and kidneys.

Rachel was a lifelong athlete, her dad said. She played field hockey at Bucknell University and exercised regularly as part of her postgrad routine. She recently moved to Charleston, South Carolina, to pursue an MBA while working a full-time remote job.

“She was always on the go, always go, and it doesn’t matter if it’s school or sports or whatever it is. She was go,” Steve said.

Rachel ran the race with the Parkinson’s Foundation in honor of her grandfather, who died of the disease, she said in a post on LinkedIn.

“My grandfather was one of my role models growing up. He taught me to stay active, push my limits, and embrace challenges,” she wrote in the post.

“I never asked her why she chose to run for Parkinson’s, but my father, her grandfather, did have Parkinson’s, and she saw how debilitating it was,” Steve said.

Steve and Michelle are now back home in Pennsylvania, trying to wrap their heads around Sunday’s events. Rachel’s two brothers are “trying to work through it,” Steve said.