A search for a missing hiker in Kings Canyon National Park has been scaled down after an extensive 10-day effort, the National Park Service said Thursday.

Quang Trong Than, 66, of Newport Beach, California, was last seen on Aug. 21 near the peak of Split Mountain on the Sierra Nevada crest, close to the boundary between the national park and Inyo National Forest, a park statement said.

Than had planned a day hike to the summit and was not carrying overnight gear. He became separated from his group at the base of the summit, the park said.

Personnel from multiple national parks, sheriff’s departments and other agencies and organizations conducted searches by ground, air and with drones.

“Despite these efforts, Than’s whereabouts remain unknown,” the park said. “Than’s itinerary was an extremely challenging one and few other hikers were in the area.”