The races involve running up and down the steep trails of Whiteface Mountain Ski Center. Runners could descend on the gondola in Saturday’s Vertical-K race, when temperatures were in the 80s, but Sunday’s more challenging Sky Race involved running both up and down. Temperatures were in the 90s Sunday.

Nick Marshak, 30, of Merrick, Long Island, collapsed around 2 p.m. Sunday, according to Jan Wellford of Keene, who organized the event through Red Newt Racing. Marshak was almost done with the race, which involves three loops: two up and down the mountain with a loop on the Flume Trails in between. Wellford said Marshak was descending the Lower Parkway ski trail on his third and final loop — about a half-mile from the finish line of the 15.5-mile race.

Wellford said he heard about Marshak’s collapse from a racer crossing the finish line, and sent a race crew member to the scene. Many other runners stopped to aid Marshak, Wellford said. When Wellford learned a few minutes later that Marshak was unresponsive, he inquired whether any friends or family members had accompanied him here for the race, but none had.

A Lake Placid ambulance crew responded and took Marshak to Adirondack Medical Center in Lake Placid, where they continued CPR and placed him in an ice bath, according to Essex County Coroner Frank Whitelaw. Attempts to resuscitate Marshak proved futile, however, and he died at the hospital. Whitelaw said the ambulance crew initially measured Marshak’s body temperature at 104 degrees, and it had risen to 108 by the time of his death. The cause of death was determined to be heatstroke.

Runners walk up a Whiteface Mountain ski trail during the Whiteface Sky Races’ Vertical-K event in Wilmington on Saturday, when the temperature was in the 80s. A runner died in a more strentuous race the following day, when temperatures were in the 90s. (Enterprise photo — Griffin Kelly)

“This guy was a trained athlete,” Whitelaw said. “He was in great shape, no underlying medical issues. We don’t know whether he was hydrated well or not.”

Wellford said he didn’t know how well hydrated Marshak was, either, but said the racer was carrying a bottle — he wasn’t sure if it was full — and had the opportunity to be hydrated at aid stations along the course.

Marshak was not in the back of the pack when he collapsed. He was in approximately 30th place, Wellford said.

Of 83 racers Sunday, 35 did not finish while 48 did. By contrast, all 86 runners finished Saturday’s less strenuous race. Wellford said the heat was the main factor in those DNFs.

Wellford said ultra-runs usually are “run all the time in all kinds of weather.” It was cold and rainy in the Whiteface Sky Races’ first year, and almost half the runners didn’t finish, he said.

The Whiteface course’s terrain is particularly difficult, which is part of its appeal among extreme runners, Wellford said.

“It’s just kind of part of the mountain running game,” he said, struggling not to sound defensive. “That said, I don’t know I want to organize any race ever again.”

Red Newt Racing has many races throughout the Northeast, but Wellford only organizes two each year: the Whiteface Sky Races and the Great Adirondack Trail Run in Keene Valley.