Millions of people visit New York’s Adirondack Park each year. Many come to hike. When someone gets lost or hurt in the vast mountain wilderness, state forest rangers, who conduct hundreds of searches annually, roll into action.

Almost all of the missions, including those with lives clearly at stake, end successfully.

But on May 10, a hiking party found human remains off an Adirondack Mountains trail. An autopsy confirmed that they belonged to Léo Dufour, a 22-year-old Canadian university student and experienced hiker whose disappearance more than five months earlier set off an all-out search.

It is not yet clear how he died, leaving a mystery: How did such a hardy young man become the rare hiker to vanish and die in the Adirondacks?

Whatever happened, it was a reminder that every hiker is always as little as “one badly sprained ankle away from a serious situation,” said Mark Scott, the owner of Great Range Mountain Guides in the Adirondacks community of Elizabethtown, N.Y.

Mr. Dufour arrived in Newcomb, N.Y., on Friday, Nov. 29, to make the roughly 18-mile round trip to the Allen Mountain summit and back in a day. The hike can take four hours or more each way. He was expected home — to Vaudreuil-Dorion, a Montreal suburb — by Saturday night.