"The police have the sad certainty that the 20 people aboard perished," police spokeswoman Anita Senti told a news conference.
There were 11 men and nine women aboard, including an Austrian couple and their son, she said.
The German-built Junker JU52 HB-HOT aircraft, dating from 1939 and nickamed "Iron Annie", was a collectors' aircraft.
It crashed into Piz Segnas, a 3,000-metre (10,000-foot) peak in the east of the country on Saturday at an altitude of 2,540 metres on the mountain's western flank, Senti said.
