There is still no sign of Mitchell Dale Stehling, a 51-year-old Texas man who has been missing since Sunday at Mesa Verde National Park.
More than 50 searchers on foot and horseback initially scoured the Spruce Tree House Trail, Spruce Canyon Trail and Petroglyph Point Trail.
Dog teams attempted to hit on his scent, and a helicopter made regular passes over 2-square-mile search area.
As of Thursday morning, 20 searchers remained, and the search will continue, said Betty Lieurance, a park information officer.
"I wouldn't say we are scaling back; rather we are giving some of our staff a rest," she said. "There is still no sign of him. We are going back over areas that are considered the highest probability for detections."
Montezuma and La Plata county search and rescue volunteers have joined the effort, and a fresh dog team was brought in.
Stehling reportedly told his wife and parents he was going to visit Spruce Tree House late Sunday afternoon. The paved trail descends for a quarter mile to the ruin. Other trails, some off limits, branch out from there. He was not carrying water, or outdoor gear, and has not been seen or heard from since.
Park officials are convinced Stehling is still in the park, and they will continue searching for him. After four days, with temperaturs reaching 102 degrees in the canyons, searchers are cautiously optimistic.
"The longer it goes, the more you wonder," Lieurance said. "Everyone is still hopeful he will be found alive."
Stehling, of Goliad, Texas, near the Gulf Coast, was last seen wearing a brown shirt and hat, khaki shorts and hiking boots. He has military experience, according to the Victoria Advocate, a newspaper near Goliad.
"Staff, archaeologists, law enforcement - everyone is out pounding the trails looking for him. They will be expanding the search area if necessary," Lieurance said.
Searchers are battling thick yucca, cactus, and pinon-juniper stands as they investigate the base of cliff faces, speculating the man may have fallen.
It was reported that Stehling does have a cellphone but it was not turned on. Cell service at Mesa Verde National Park is generally nonexistent, but there are some spotty areas near the road and higher points where a signal can be obtained.
Mesa Verde officials did not know whether the man had any backcountry experience or had medical issues. He was not carrying any overnight gear.
A backcountry gate near Spruce Tree House ruin offers access to Spruce Canyon and connects to a network of off-limit trails only available for special tours. There are stashes of survival gear, including water and food, at certain points.
The searchers followed up on some clues early on, following footprints, and finding a water bottle, but they turned out to be unrelated to the missing person, Lieurance said.
"It has been hot, and he is not accustomed to high altitudes," she said.
Posters alerting visitors to the park about the missing man have been put up, and hikers are being advised to be on the lookout.
"It is pretty unusual for someone to be missing this long because the backcountry is off-limits," Lieurance said. "Usually, they turn up in two to five hours or are found on closed trails by rangers."
According to the Victoria Advocate newspaper in Texas, the family has high hopes that Stehling will be found safe.
Melissa Stehling told the newspaper that he comes from a military family and has the wits to survive.
"He's probably already built a tree house and is waiting for rescuers to find him," she said.
A post on Facebook by his daughter, Haley Stehling Wagner stated: "Thank you everyone for your thoughts and prayers. Please continue to pray for may dad that he is found safe soon."