A participant in a grueling 250-mile ultramarathon on trails across northern Arizona died Tuesday after experiencing a medical emergency, race organizers and local law enforcement said.

First responders attended to a woman in her 40s who was participating in the race and collapsed at a trailhead in the Groom Creek community south of Prescott, Yavapai County Sheriff’s Office spokesperson Paul Wick said on Wednesday.

Cocodona 250 ultramarathon codirector Erika Snyder confirmed the death and declined to provide further details at the request of the runner’s relatives.

Ultramarathons can test the boundaries of human endurance, but deaths are very rare relative to the total number of participants in a sport that has in recent years grown in popularity around the world, with races hosted from Death Valley to the Himalayas.

The Cocodona is one of the toughest ultramarathons in the U.S., featuring a course that rises more than a mile in elevation — twice — while passing through Sedona from its start in Black Canyon City.

This year’s race started Monday before dawn, and the overall winner, Rachel Entrekin, finished Wednesday afternoon to a roaring crowd in Flagstaff while setting a course record.

Organizers say they’re moving to a lottery registration system next year because of the race’s popularity.

Aid stations provide opportunities to eat, hydrate and adjust equipment, with a team of medical professional stationed across the course, Snyder said.

Racers often nap on the ground along the way in a race that can last five days.

In 2021, freezing rain and high winds were linked to the deaths of 21 runners at an ultramarathon through mountains in northwestern China.