JACKSON (WNE) — Yellowstone National Park is winding down search and recovery efforts for Kim Crumbo, 74, who had been on a four-night backcountry trip to Shoshone Lake with his brother, who died of hypothermia.

The National Park Service has been searching for Crumbo for nearly three weeks using helicopters, boats, sonar technology and ground crews, a Friday news release said. Deteriorating conditions were expected over the upcoming week, including snow and freezing temperatures, but the park said it would continue limited search efforts as long as conditions allow.

After family reported the men overdue, search crews found the body of Mark O’Neill, of Chimacum, Washington, on the lake’s east shore Sept. 20. Crumbo has not been located.

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

“All of us at Yellowstone extend our deepest sympathies to the families, friends and colleagues of both Mark and Kim,” Superintendent Cam Sholly said in a statement.

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