LEADVILLE, Colo. (AP) — An experienced cyclist is dead after participating in the grueling Leadville 100 mountain bike race in Colorado.

Fifty-five-year-old Scott Ellis died Saturday near the end of the 100-mile, high-altitude course.

His Peloton-Specialized team said he apparently suffered a heart attack.

Lake County coroner Shannon Dent said natural causes seem to be to blame but wouldn’t elaborate, referring questions to the race’s organizers, Life Time Fitness. A spokeswoman, Amy Henderson, said the company would not share details of Ellis’ death out of respect for him and his family.

The race is nicknamed the “Race Across the Sky” and draws about 1,500 riders a year. Past participants have included Lance Armstrong and Levi Leipheimer.

Ellis was competing in his 19th Leadville 100. His death is the first in the race’s 32-year history.