An autopsy revealed that cardiac arrest killed a well-conditioned trail runner last month from Eagan who collapsed while competing in a rigorous North Shore race, authorities said Tuesday.

Jon Mathson, 41, collapsed May 20 in Lutsen during the 25-kilometer (15.5-mile) Superior Trail Race a few miles from the finish at Caribou Highlands Lodge.

The Midwest Medical Examiner’s Office in Anoka found that Mathson died from “natural cardiac arrest during exertion due to … coronary artery disease” brought on by a hardening of the arteries, the Cook County Sheriff’s Office said in a statement.

Mathson, a science teacher at Eagan High School since 2007 who also supervised the school weight room, “had no known [adverse] medical history,” the statement noted.

Adam Schwartz-Lowe, a friend of Mathson’s and a fellow trail runner competing in the same race, said he knew of no health problem with Mathson. “He was easily capable of running the distance and enjoying it,” Schwartz-Lowe said.

Three times Mathson had previously completed the extremely hilly and rugged out-and-back route that traverses the Sawtooth Mountain Range on the Superior Hiking Trail in the far reaches of northeastern Minnesota.