iStock/Thinkstock(ZURICH) — The airline whose vintage World War II plane crashed into a mountain in southeastern Switzerland, killing all 20 people on board, said it was “deeply saddened” and has suspended all flight operations until further notice.

The crash during a sightseeing trip on Saturday killed all of the plane’s 17 passengers and three crew members.

The passengers were traveling from Locarno back to Duebendorf, near Zurich, after a two-day trip in Switzerland’s southern Ticino region. Of the passengers, 14 were Swiss and three were Austrian, the operator, Ju-Air, said.

“The Ju-Air team is deeply saddened and thinks of the passengers, the crew and families and friends of the victims,” the operator said in a statement, adding that a helpline has been set up for relatives of the victims.

The cause of the crash is unknown. Ju-Air said that the aircraft, built in 1939, flew 10,187 hours before the crash and that it underwent regular maintenance. It had to be serviced after every 35 hours of flying, and the last regular maintenance took place in late July, the operator said. The plane underwent its full annual service in the winter, and there were no known technical problems with the aircraft, Ju-Air said.

The two pilots, a 63-year-old and a 62-year-old, had extensive experience flying the vintage plane, Ju-Air said. The 62-year-old was an airline pilot for more than 30 years while the 63-year-old also had 30 years of experience as an airline pilot and as a military air force pilot, Ju-Air said.

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