Two climbers were waiting to be rescued near the peak of Denali, a colossal mountain that towers over miles of vast tundra in southern Alaska, officials said Wednesday. Originally part of a three-person team that became stranded near the top of the mountain, the climbers put out a distress call more than 30 hours earlier suggesting they were hypothermic and unable to descend on their own, according to the National Park Service.

Weather conditions made attempts to rescue the climbers particularly treacherous this week, the park service said. Cloud cover posed dangers to aviation and ground search crews who were unable to reach the upper part of Denali between 1 a.m. local time on Tuesday, when park rangers received the climbers’ initial satellite call, and 9 a.m. on Wednesday, when the National Park Service said rescuers were “waiting for clouds and windy conditions to dissipate on the upper mountain.” 

CBS News contacted the National Park Service for updates on the rescue mission Thursday morning but did not receive an immediate response.

Standing 20,310 feet at its tallest point, Denali is the centerpiece of a rural and massive namesake national park and holds the record for being the highest peak in North America. The sprawling national park and the mountain itself are some of southern Alaska’s main tourist draws, which together attract around 600,000 visitors every year. Many who travel to the national park never actually see Denali, though, because clouds in the region can be so thick that they completely obscure the mountain despite its size.    

Denali park rangers communicated with the group of climbers for several hours after receiving the SOS through InReach, a portable device that uses satellite to send messages and has a GPS system that allows recipients to see its location. Although the group had told rangers at around 3:30 a.m. that they planned to climb around 700 feet down Denali to a plateau called the “Football Field,” they did not continue communicating from then on and their location higher up appeared to remain the same, according to the National Park Service.

A high-altitude helicopter and, later, a plane launched by the Alaska National Guard, searched the mountain and did locate two climbers while flying overhead on Tuesday. A climbing guide found the third near a lower elevation, at about 18,600 feet above ground, and along with a team of people helped that person descend another 1,400 feet or so to a camp where rescue crews were waiting. The National Park Service said that climber suffered severe frostbite and hypothermia. 

Their helicopter finally retrieved that person at 10:15 p.m. Tuesday and transferred the climber to a LifeMed helicopter in Talkeetna, the nearest major town. The helicopter also evacuated two other climbers being treated for frostbite in a medical tent on the mountain.

Although the National Park Service said “an experienced expedition guide” was able to reach the two other climbers, who made it by the end of the day Tuesday to the Football Field in upper Denali, that guide had to return to a lower point later in the night as clouds moved back in “for his own safety and for the safety of his team.”

With its stark and unusually challenging landscape, Denali has become a popular climbing spot for ambitious mountaineers. The National Park Service said that Memorial Day weekend often marks the start of the busiest weeks of the year on the mountain, and about 500 people were attempting climbs as of Wednesday. 

Around 15% of climbers reach the summit of the Denali, according to the park service, and some have died trying. Earlier this month, the Associated Press reported that a climber was found dead about 18,000 feet up the mountain while attempting a solo ascent.