BERN, Switzerland — A propeller plane built at the onset of World War II crashed in the Swiss Alps this weekend while on a sightseeing tour, killing all 20 people on board, the Swiss police said on Sunday.

The German-built Junkers Ju-52 was carrying 17 passengers, all Swiss except for an Austrian couple and their son, the Swiss authorities said. Three crew members were also killed in the crash on Saturday.

The plane was on an Alpine sightseeing tour from Locarno, in the Italian-speaking region of southern Switzerland, to Dübendorf, an airport near Zurich, when it crashed into the mountainside near the ski resort of Flims, in eastern Switzerland.

The authorities said that an investigation was underway, but they ruled out an explosion on board or an in-flight collision as the cause of the crash.

The crash occurred shortly after another small plane crashed into the Swiss Alps, in the canton of Nidwalden, killing a family of four on board. The Swiss president, Alain Berset, offered condolences on Sunday on behalf of the government to the relatives of the 24 people killed in the two plane crashes.

The Junkers plane struck the Piz Segnas Mountain at an altitude of about 8,000 feet. It plunged down “almost vertically, at high speed,” Daniel Knecht, a spokesman for the Swiss transport safety investigation board, said at a news conference on Sunday.

The plane was operated by Ju-Air, a Swiss company that offers tours with vintage former Swiss military aircraft. The Ju-52, also known as “Aunt Ju,” was rolled out by Junkers, the German plane manufacturer in the 1930s, initially with a single engine but then as a three-engine aircraft.

It was used by the Luftwaffe, the aerial branch of the German forces during World World II, both as a transport plane and as a bomber. It first came into military use during the Spanish Civil War, notably as part of the German-led bombing of the Basque town of Guernica in 1937, which was later immortalized by Picasso in his famous antiwar painting.

During World War II, the Nazis also used the plane, including to help supply their campaign in North Africa. The Swiss Air Force decommissioned its last three Ju-52 planes in 1982, which were then taken over by an association of Swiss aficionados of such vintage aircraft.

Kurt Waldmeier, a former pilot and the president of Ju-Air, called it “the most tragic day in the history” of his company, but said it was too early to draw conclusions about what had caused the crash. “Nobody has more interest than Ju-Air in clarifying the events, so that such an accident can never occur again,” he said.

Mr. Waldmeier told a news conference on Sunday that the plane, built in 1939, had flown more than 10,000 hours and had been regularly checked because of its old age.

The last inspection was in late July, and the aircraft had no history of technical problems, he said. The plane was navigated by using sight and maps rather than modern instruments, but all three members of the crew had more than 30 years of professional experience.

The 62-year-old captain, who was not immediately named, had spent three decades as a commercial pilot for the national carrier, Swissair, and its successor, Swiss. He had been flying the Ju-52 regularly since 2004.

Rescue teams and investigators, with the support of helicopters, were sifting through the debris on Sunday. In recent days, Switzerland has been hit by a heat wave that has pushed daytime temperatures above 30 degrees Celsius (86 degrees Fahrenheit).

While flight specialists told Swiss news outlets that intense heat could make it more difficult to fly such an aircraft, they said it was too early to suggest that high temperature played any part in the crash.