"One can ascertain that the aircraft hit the ground almost vertically at high speed", Daniel Knecht, who heads the aviation division of the Swiss Transportation Safety Investigation Board, said.
"The police have the sad certainty that the 20 people aboard perished", police spokeswoman Anita Senti told a news conference.
The doomed aircraft had departed from Ticino in southern Switzerland and was bound for Duebendorf military airfield near the city of Zurich when it crashed into the mountainside at an altitude of 2,450 meters (8,000 feet) above sea level on the west side of the Piz Segnas mountain shortly before 5 p.m. local time (1500 GMT).
Police said they are not aware of any distress call from the aircraft before it crashed.
Police said Sunday they had determined that the 17 passengers and three crew members on board the plane all died.
The vintage aircraft crashed into the Piz Segnas mountain in the Flims resort on Saturday.
The area, which includes a glacier, is popular with hikers.
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Wreckage of JU-Air plane involved in crash in the Swiss Alps.
Authorities are investigating what caused the deadly crash.
The company's flight operations were suspended, it said.
JU-Air says it runs a small fleet of four Junker planes, all built in 1939, which are for hire.
Chief executive and co-founder Kurt Waldmeier said the planes "are flown exclusively by very experienced professional pilots, and strictly checked and maintained by our own technicians". The JU-Air airline operated three of the vintage planes - decommissioned by the Swiss Air Force and affectionately known as "Auntie Ju" planes - as part of sightseeing tours, Reuters reported.
The company did not offer any further information.