A 51-year-old Colorado man died after suffering a sudden medical emergency during a grueling Ironman competition in Wisconsin over the weekend.

The athlete, whose name has not been publicly released, was competing in the Ironman 70.3 Wisconsin on Saturday when something went terribly wrong during the swimming portion of the race.

He was pulled from the water after becoming unresponsive and had no pulse when emergency crews reached him.

Madison Fire Department spokesperson Cynthia Schuster said first responders immediately began trying to save his life.

“At the time, they had a patient who did not have a pulse and wasn’t breathing, so life-saving measures began,” Schuster said.

Exactly what caused the medical emergency remains unclear.

“We don’t know why he went unresponsive or why he didn’t have a pulse,” Schuster said.

Race organizers later confirmed the athlete had died and issued a somber statement.

“We are deeply saddened to confirm that an athlete passed away during today’s IRONMAN 70.3 Wisconsin event after experiencing a medical emergency,” organizers wrote on Facebook.

“We extend our heartfelt condolences to the athlete’s family, friends, and loved ones during this difficult time.”

The Ironman 70.3 Wisconsin is a punishing endurance event that begins with a 1.2-mile swim in Lake Monona.

Athletes then take on a 56-mile bike ride through Dane County before finishing with a 13.1-mile run that ends near the Wisconsin State Capitol.

Authorities have not yet announced what caused the 51-year-old athlete’s death.