According to Denver Post,

David Cook was reported missing on September 20, 2016, after not returning from a solo climb.

During an investigation into Cook’s disappearance, deputies learned that Cook had arrived at the area shortly after 11a on Monday, September 19, 2016, the release said.

“Deputies learned that Cook had been seen on Monday (September 19) hiking near the trail to Maroon Peak,” according to the sheriff’s office. “Additionally, Cook’s cell phone pinged cell towers in Aspen and Gunnison County later in the day on Monday. It is not unusual for cell phones at high elevations to ping cell towers from the wilderness.”

Cook was also reported to have been seen, investigators say, on the morning of September 20, 2016 by a U.S. Forest Service employee near Maroon Lake, which was his last confirmed sighting.

Cook is described as a former Marine and expert mountaineer with a breadth of experience in the backcountry.