He said hot weather conditions did not contribute to the crash and that the aircraft fell nearly vertically out of the sky at relatively high speed.
"The police have the sad certainty that the 20 passengers perished", said a spokesperson for regional police in the canton of Grisons.
The aircraft crashed about 2540m above sea level, on the western side of a 3000m peak called Piz Segnas.
Twenty people are confirmed dead after a vintage World War II aircraft crashed into a Swiss mountainside, police reports said.
Police said those killed were couples from the Swiss cantons of Zurich, Thurgau, Lucerne, Schwyz, Zug and Vaud, along with a three-member family from Austria and three crew members from Thurgau and Zurich.
The crash occurred above the Swiss Alpine resort of Flims, an area popular with hikers and skiers.
Officials appeared dubious about suggestions that unusually hot weather in Switzerland, like other parts of Europe, might have been a main cause of the crash. The heat makes air thinner, increasing the burden on the aircraft.
"Older planes, if they are correctly maintained, can be operated safely", he said.
Junkers JU-52 planes were manufactured between the 1930s and 1950s.
Ju-Air started operating flights with the old-timers in 1983, and the plane that crashed - with the registration HB-HOT - had been in service with the company since 1985.
According to JU-AIR, a Ju-52 airplane, which seats 17 passengers along with two pilots, had crashed.
Each of the pilots flying it had over 30 years of experience in both the military and civilian sector.
The aircraft was in possession of JU-Air, a company with links to the Swiss Air Force.
The aircraft have three engines, one on the nose and one on each wing. JU-AIR also suspended flights with the remaining two Ju-52s until further notice.