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Cause of death determined for man found in Yellowstone National Park, recovery effort continues for brother

Grand Teton National Park’s interagency helicopter at Shoshone Lake
Shoshone Lake in Yellowstone National Park
Posted at 11:07 AM, Sep 29, 2021
and last updated 2021-09-29 13:13:09-04

GREAT FALLS — Yellowstone National Park said in a news release on Wednesday, September 29, 2021, that an autopsy has been completed on 67-year-old Mark O’Neill, whose body was found on the east shore of Shoshone Lake in the park on Monday, September 20.

The autopsy determined the cause of death of O’Neill was exposure (hypothermia).

O’Neill, from Chimacum, Washington, along with his brother, 74-year-old Kim Crumbo from Ogden, Utah, were reported overdue by a family member on Sunday, September 19, from their four-night backcountry trip to Shoshone Lake.

Park crews found a vacant campsite with gear on the south side of Shoshone Lake, as well as a canoe, paddle, personal flotation device, and other personal belongings on the east shore of the lake.

Search and recovery efforts continue at the lake to locate Crumbo. Crews from the National Park Service’s Submerged Resources Center are using sonar equipment to detect clues in the water.

Park crews continue to search for Crumbo by foot and boat, with assistance from Grand Teton National Park’s interagency helicopter and dog teams from Western Montana Search Dogs. These recovery efforts will continue for the next several days as conditions warrant.

O’Neill and Crumbo are both National Park Service retirees, and Crumbo is a former Navy SEAL.

Shoshone Lake, the park’s second-largest lake, is located at the head of the Lewis River southwest of West Thumb. The average year-round temperature of the lake is about 48 F (9 C). Survival time is estimated to be only 20 to 30 minutes in water of this temperature.

If you have information that could help investigators piece together a timeline of events, or if you were in the Shoshone Lake area between September12 and19, you're asked to contact officials at Yellowstone at 307-344-2428 or

The news release also says: "This incident remains under investigation. While we cannot comment further about the specifics of this investigation, we will provide updates when appropriate to do so."

Park officials also noted that Crumbo and O'Neill are brothers, not half-brothers as they had initially reported.