SANDPOINT - A 15-year-old skier who was found unresponsive in a tree well in Schweitzer Mountain Resort's north bowl last December likely died of an undetected heart condition.

Coroner Kitt Rose listed William Clyde Johnson's cause of death as a "probable fatal cardiac arrhythmia" and the manner of death as "natural," according to a copy of her report obtained by The Bonner County Daily Bee.

"It's just one of those bizarre things from left field that you have no idea he even had. It's nothing that the doctors missed; it's just that there was never any way to even suspect it," said Johnson's mother, Susan.

Susan Johnson said an autopsy revealed that her son had an anomaly in his left descending coronary artery.

"My son was the picture of health - he ran cross country, skied, snowboarded and wakeboarded," Johnson said. "It came as a huge surprise. We don't have anything like that in the family."

The Sandpoint High School student athlete was found unresponsive in a tree well, sparking suspicion that he suffocated after falling head-first into the dangerous void at the base of the tree.

Tree wells form around the bases of evergreen trees when low-hanging branches prevent snow from consolidating around the tree trunk, creating a hole that can entrap skiers and snowboard riders.

Sheriff's officials said Johnson was discovered by snowboarders riding in the north bowl shortly after noon on Dec. 22, 2009.

Heavily redacted sheriff's reports obtained through Idaho's public records law indicate that the passersby found Johnson with his legs sticking out of the tree well. They pulled Johnson free, but it appeared that he had already died.

Ski patrollers were promptly summoned, but efforts to revive Johnson were unsuccessful.

The deputy's reports said Johnson was found in an in-bounds area known alternately as ABC Trees and ABC Rocks, a steep and densely treed area below the north ridge. The area holds some of the resort's most technically challenging terrain, though those who knew Johnson described him as an experienced skier and snowboard rider, according to the police reports.

Will Johnson, the reports said, was skiing with others but they were ahead of him when he fell.

Susan Johnson finds some degree of solace in the likelihood that her son did not suffocate upside down in the snow. The tragedy also directed some attention to the perils tree wells pose to those traversing steep and deep alpine environments, she said.

"Hopefully that will be some good that comes out of this - some education," she said.