BERLIN — An old-time propeller plane crashed near-vertically at high speed into a Swiss mountain, killing all 20 people on board, police said Sunday.

Swiss officials said they did not know the cause of the crash and said they were not aware of any distress call from the plane before it crashed the Piz Segnas mountain above the Alpine resort of Flims.

Nine women and 11 men were killed, most from Switzerland but also a couple and their adult son from Austria.

The victims were between age 42 and 84, officials added.

"The aircraft plunged almost vertically at high speed into the ground,” said Daniel Knecht, an official from the Swiss Transportation Safety Investigation Board.

Speaking at a news conference Sunday in Flims, Knecht said the plane did not have a black box on board. He added that officials have essentially ruled out a collision with another aircraft or hitting an obstacle such as a wire.

He also said there's no indication of foul play or that the aircraft lost parts or broke up before the crash.

Officials said they expect the investigation into the cause of the crash to be "relatively complex."

The plane was flying the passengers back from a two-day trip in southern Switzerland to its base near Zurich.

The plane was operated by Ju-Air, a Swiss company that offers tours with vintage former Swiss military aircraft.

The aircraft have three engines, one on the nose and one on each wing.

The company, which operates two other Ju-52s, suspended flights until further notice after the crash.