Tragedy struck on Mt. Everest over the weekend as at least four mountaineers died during a very crowded couple of days near the summit of the world’s tallest peak.

The climbers have been identified as Eberhard Schaaf, 61, a German doctor; Shriya Shah, 33, of Canada; Song Won-bin, 44, of South Korea, and Ha Wenyi, 55, of China.

A Sherpa guide remains missing.

The first clear weather of the season led to a mass summit attempt on Friday and Saturday, and about 150 climbers reached the top the 29,035-foot Himalayan giant, mostly on Saturday.

But ferocious high-mountain winds and large crowds that choked ascent routes on Everest’s South Col route, delaying summit bids of several teams, created an extremely dangerous situation.

“There was a traffic jam on the mountain on Saturday. Climbers were still heading to the summit as late as 2:30 p.m. which is quite dangerous,” Gyanendra Shrestha, of Nepal’s Mountaineering Department, told the Associated Press.

“With the traffic jam, climbers had a longer wait for their chance to go up the trail and spent too much time at higher altitude. Many of them are believed to be carrying limited amount of oxygen and not anticipating the extra time spent.”

Ang Tshering Sherpa of the Kathmandu-based Asian Trekking adventure company explained to Agence France Presse: “Climbers spend their energy on the ascent and they are exhausted and fatigued on the descent.”

Altitude sickness and exhaustion are believed to have been primary factors in the fatalities. The Seoul-based Yonhap news agency is reporting that Song, after collapsing with fatigue, fell at an area known as The Balcony, a small platform area at 27,600 feet.

AFP reports that the body of the Chinese climber was found at 28,000 feet.

Mt. Everest’s so-called “Death Zone” is the region above 8,000 meters, or 26,000 feet, where freezing temperatures and low oxygen levels make it nearly impossible to survive for more than 48 hours.

On average, a handful of climbers perish each year on Mt. Everest. This year, according to Everestnews.com, there have been at least six fatalities, and 198 successful summits.

The most tragic day on Everest, with late summit bids also a contributing factor, was May 10, 1996, when eight people died after becoming trapped in a snow storm.

That season was the deadliest on record, with 15 fatalities.

Nearly 4,000 people have reached the top of Everest since 1953, when Sir Edmund Hillary and Sherpa Tenzing Norgay became the first to scale the peak.

– Top image shows Mt. Everest as viewed from Nepal, while second image shows the southern route to the summit