The following is a release from the National Park Service –

An autopsy has determined that a man whose body was found on the shore of a lake in Yellowstone National Park died of hypothermia, park officials said Wednesday.

Mark O’Neill, 67, of Chimacum, Washington, had been on a four-night backcountry trip to Shoshone Lake with his brother, Kim Crumbo, 74, of Ogden, Utah. A relative reported them overdue Sept. 19.

Searchers found O’Neill’s body Sept. 20 on the lake’s eastern shore. A search for Crumbo on foot and by boat will continue, weather permitting, for several days, park officials said in a statement.

A vacant campsite was found on the lake’s south side. What happened remains under investigation, they said.

The men were experienced boaters and former National Park Service employees, and Crumbo is a former Navy SEAL who served in Vietnam.

Crumbo co-founded the conservation group Grand Canyon Wildlands Council, which later merged with another group to form Wild Arizona.

Shoshone Lake covers 13 square miles (33 square kilometers) and has an average temperature of 48 degrees Fahrenheit (9 Celsius). People without insulating gear typically can survive only 20 to 30 minutes in such cold water, officials said.