The landslide, which struck on Wednesday, sent mud, rocks and dirt flooding down the Piz Cengalo mountain into the outskirts of the village of Bondo, near the Italian border.
About 100 people were evacuated, some airlifted out by helicopters.
Police in the canton of Graubunden, which initially announced no one was hurt, confirmed Thursday that eight hikers, from Germany, Austria and Switzerland, had been in the region of Val Bondasca where the landslide occurred and were missing.
Another group of five to six people had also been reported missing, Andrea Mittner of the Graubunden police told a news conference, adding though that it was unclear if they had actually been in the affected area at the time of the landslide.
"Perhaps they had already left" the region, he said.
No children were believed to be among the missing, he said.
He said a massive search-and-rescue operation was scouring a five-square-kilometre (three-square-mile) area, involving some 120 emergency workers, helicopters and rescue dogs.
The landslide set a full four million cubic metres (141 cubic feet) of mud and debris in motion, its relentless moving mass stretching 500 metres (1,640 feet) across, according to the regional natural hazards office (AWN).
The event was so severe that the vibrations set off seismometers across Switzerland, which measured the equivalent of a magnitude 3.0 earthquake, according to the Swiss Seismological Service.
Images showed an unstoppable mass of thick mud and sludge moving down the mountainside like lava, tearing up trees and demolishing at least one building in its path and partially engulfing others.
She said she first noticed something was amiss when a large flock of birds began flying nervously around the church tower, and then an ear-deafening bang that sent rocks pouring down the mountain.
"There were so many. It did not stop," she said, adding that a blanket of smoke and dust soon covered Bondo.
"You could not see anything for an hour. But you could still hear stones falling down. It smelled of granite," Nunzi said.
Police said 12 farm buildings, including barns and stables, had been destroyed, while Graubunden's main southern highway was closed to traffic.
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