The body of a 67-year-old man was found along the shore of one of Yellowstone National Park’s backcountry lakes on Monday, and park officials say another man remains missing.

The two men — identified as the late Mark O’Neill of Chimacum, Washington, and his half-brother Kim Crumbo, 74, of Ogden, Utah — had taken a backcountry trip to Shoshone Lake. They planned to spend four nights on the excursion, but a family member reported them as overdue on Sunday.

That same day, Yellowstone crews found a vacant campsite on the south side of the lake, with a canoe and other gear on the eastern side. Then on Monday morning, searchers found O’Neill’s body along the east shore.

On Tuesday, search and rescue crews were continuing to look for Crumbo. Some 10 crew members were searching the area on foot, with personnel from Grand Teton National Park aiding the search from the air, the National Park Service said in a release.

“This incident remains under investigation,” Yellowstone spokeswoman Morgan Warthin said in a release. “While we cannot comment further about the specifics of this investigation, we will provide updates when appropriate to do so.”

In the meantime, Warthin asked the public “to maintain distance from any law enforcement personnel, equipment, vehicles and their related activity for the safety of the public in this remote area and to protect the integrity of our work.”

Shoshone Lake, located southwest of West Thumb, is believed to be the largest lake in the lower 48 states that isn’t reached by road. It spreads across 8,050 acres and the water has an average year-round temperature of about 48 degrees Fahrenheit, the Park Service said.

“Survival time is estimated to be only 20 to 30 minutes in water of this temperature,” Warthin wrote.

Both O’Neill and Crumbo are Park Service retirees, the release said, with Crumbo having served as a Navy Seal.