Twenty-four people were killed in two separate aircraft crashes in Switzerland on Saturday.

A large-scale operation was initiated on Sunday morning after the World War II vintage Junkers JU-52 aircraft crashed into the Piz Segnas mountain in Graubunden canton in the eastern part of the country. Five helicopters were deployed and the airspace around the accident site was closed down along with some hiking trails, the Swiss Info news website reported.

There were 17 passengers, two pilots, and a flight attendant on board the aircraft, which is owned and operated by the JU-AIR company in Dubendorf, near Zurich. The company specialises in sightseeing flights using old military planes.

“The police have the sad certainty that the 20 people aboard perished,” AFP quoted police spokesperson Anita Senti as saying. All the passengers, except an Austrian couple and their son, were Swiss.

The cause of the crash is not yet known, and officials said they are not aware of the plane having sent any distress call. Swiss Transportation Safety Investigation Board official Daniel Knecht said the plane may have hit the ground almost vertically and at high speed. It reportedly did not have crash-resistant cockpit voice and data recorders that more modern aircraft have, he added.

A few hours before the crash in Graubunden, a family of four was killed when their small plane crashed in a forest in central Switzerland’s Nidwalden canton, reported Reuters. The police said the plane had taken off from Kagiswil airport after 9.30 am, and was on its way to France, reported AFP.