Four people were killed and five others injured Sunday morning in an avalanche in the village of Irihirose in Niigata Prefecture, police said.
The avalanche occurred shortly after 8:30 a.m. at Mount Asakusa in the village.
Two policemen and one firefighter were among the four killed, police said. The nine were part of a group of 43 searching for a man who had gone missing while collecting edible wild plants.
The victims were identified as police officers Yoshinori Odaira, 31, and Hiroaki Sano, 35; firefighter Saburo Isobe, 52; and hotelier Otoichi Asai, 73.
Asai was known among Japan's mountaineers as a veteran guide on who had helped rescuers negotiate the mountain for decades.
The nine had found the body of Fumio Tsuchida, 52, on the mountain and were waiting for a helicopter to bring it down when the avalanche hit, police said.
The avalanche started as small pieces of snow and ice rocks but soon progressed in size as snow blocks the size of tatami mats started racing down, police said.
The avalanche occurred eight minutes after the helicopter approached the area, and investigators are checking to see whether it triggered the avalanche, police sources said.
Police said the area remains buried in several meters of snow, which is unusual at this time of year.
But temperatures soared to nearly 30 on Thursday and Friday at the base of the mountain and marked 23 on Saturday, despite the rain, according to the local meteorological observatory.
The affected area is locally known as Yasunosawa Valley, a place where many people visit to pick wild plants in the spring.