Daniel Knech of the Swiss safety investigation service SESE said the crew did not have time to send out a distress signal.
Authorities have provided few details about those on board JU-Air's plane which crashed on the west side of the Piz Segnas mountain in the canton of Graubuenden on August 4.
Police in central Switzerland said that a small plane has crashed in a forest, killing several people.
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.
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The vintage aircraft crashed into the Piz Segnas mountain in the Flims resort on Saturday.
Almost 5,000 Ju-52 planes were manufactured between 1932 and 1952.
The wreckage was discovered engulfed in flames, and the cause of the crash is now being investigated by the Federal Prosecutor's Office, in cooperation with local Swiss authorities.
The airline posteda statement online: "We have the sad duty to announce that one of our Ju-52 aircraft had an accident today".
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A plane crashed on the Piz Segnas mountain.
The aircraft belongs to JU-Air, a company with links to the Swiss air force, the ATS news agency reported. Its pilots are ex-military and professional pilots, all of them volunteers.
JU-Air runs sightseeing tours on its squadron of German-made vintage aircraft.
The company, which operates two other Ju-52s, suspended flights until further notice after the crash.
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