By Manesh Shrestha, for CNN
updated 7:27 PM EDT, Mon May 21, 2012

Kathmandu, Nepal (CNN) -- Four people died while coming down the southern slope of the mountain during the weekend after reaching Mount Everest's 8848-meter (29,028 foot) summit, officials said.

The victims have been identified as Ebehard Schaaf, 61, a German medical doctor; Sriya Shah, 33, a Nepali-born Canadian woman; Song Wondin, a 44-year-old man from South Korea; and Wen Ryi Ha, 55, of China, according to officials with the tourism and civil aviation ministry and at the base of the mountain.

"Climbers climbing down the mountain have said that they have seen the body of the Korean," said Tilakram Pandey, of the tourism and civil aviation ministry, by phone from the base of the mountain.

The Korean had earlier been reported missing. There were reports of a Nepali missing as well, but those reports could not be verified, Pandey said.

David Breashears, a climber and filmmaker who has reached Everest's peak five times, told CNN that anyone still unaccounted for after nightfall has poor prospects for survival.

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"You will surely perish at night at those elevations and those temperatures without the safety of a tent and the protection of a warm sleeping bag," Breashears told CNN.

The cause of the German's death has been diagnosed as high-altitude cerebral edema, according to Ang Tshering Sherpa of Asian Trekking, which organized his expedition.

Mountaineers often expend all their energy while ascending the mountain and do not think about energy needed to come down. "As a result, they become weak and suffer from altitude sickness," Sherpa said.

What is Altitude sickness: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001190/

And Breashears said a "tremendous number" of climbers attempt to scale Everest at this time of year, sometimes leading to delays atop the mountain.

"It's a period when all the camps are in, all the supplies are in, the fixed ropes are ready and they're waiting for an abatement of the jet stream winds," he said.

Saturday was also windier than usual. On Saturday a 73-year-old Japanese woman, Tamae Watanabe, had climbed the mountain from the northern side on the Tibet-China border to become the oldest woman on the summit.

Bal Krishna Ghimire, a spokesman of the tourism and civil aviation ministry, said that mountaineers began summiting the mountain from the south side this spring season only since Saturday, about 10 days later than usual, because of adverse weather conditions.

The spring mountaineering season, which lasts from March 1 to May 31, is the most popular season to climb the Himalayan peaks in Nepal.

With these four deaths the number of people killed on Everest this year has reached six, ministry official Dipendra Poudel said. Two Nepali sherpas died on the mountain earlier this month.