A competitor in the Ironman US Championship died after struggling during the swim portion of the race in the Hudson River between New York and New Jersey, a race spokeswoman said.
The cause of death was not immediately clear, and race organizers were awaiting autopsy results, Emily Vicker, a spokeswoman for the Aquadraat Sports Ironman US Championship, said in a statement.
The competitor's name was not immediately released.
"The athlete experienced distress during the swim portion of the race, received immediate on-site medical attention and was transported to a nearby hospital," Vicker said.
Jordan Rapp won the Ironman competition on Saturday with a time of eight hours, 11 minutes and 18 seconds, according to the website Ironman.com.
In the women's competition, Mary Beth Ellis won with a time of nine hours, two minutes and 48 seconds, the site said.
The swim portion of the event had been threatened earlier this week when a sewer line broke around Tarrytown, about 30 miles (48 kilometres) north of New York.
Several million gallons of chlorinated sewage were discharged into the Hudson River as part of the repairs.
On Thursday health officials said the river was likely clean enough to host the race.
The 2.4-mile (3.9-km) swim was followed by a 112-mile (180-km) bicycle ride on New York and New Jersey highways and a 26.2-mile (42-km) run ending in Manhattan.