ZURICH (Reuters) - JU-Air plans to resume flying on Aug. 17, the airline said on Tuesday, two weeks after one of its small vintage planes crashed in the Swiss Alps with the loss of all 20 people aboard.

A Junkers Ju-52 airplane is seen in the logo of local airline JU-AIR at its headquarters at the airport in Duebendorf, Switzerland August 7, 2018. REUTERS/Arnd Wiegmann

The crash occurred on Saturday when one of the airline’s midcentury JU-52 aircraft went into a nosedive on the west side of the Piz Segnas mountain in the canton of Grissons.

The cause of the incident -- which occurred hours after a family of four was killed when their small plane went down further west in the Alps -- remains under investigation and could take weeks or months to determine, authorities said.

The effort is complicated by the vintage plane’s lack of a “black box” flight recording device.

“We plan to resume flights on August 17 unless we receive any indications until then that safety might be a concern,” JU-Air spokesman Christian Gartmann said. “At the moment we don’t have any indication that it would be unsafe to resume flight.”

The country’s aviation authority, the Federal Office of Civil Aviation, said that so long as there is no firm suspicion of a technical defect, it cannot order the grounding of the airline’s fleet.

The airline would need to ensure its crew and maintenance staff were psychologically fit for flight, the federal office warned, adding it would take additional measures, including issuing a temporary flight ban if necessary, should pertinent information surface meanwhile.

The plane hit the ground nearly vertically and at high speed, authorities determined. It was heading north past the Martinsloch rock window, when -- instead of flying over the ridge -- it made a sharp curve and suddenly tilted into a nosedive, an eyewitness said.

“It didn’t take 15 seconds,” eyewitness Raini Feldner told Swiss newspaper Tages-Anzeiger.

Established in 1982, JU-Air offers sightseeing, charter and adventure flights on mid-century Junkers Ju-52 aircraft decommissioned by the Swiss Air Force and known affectionately in German as “Auntie Ju” planes.

The crash of the aircraft, built in the late 1930s as a military aircraft, was the first time the airline had had an accident resulting in death or injury to passengers or crew members, the airline’s chief executive said.

Recovery works were wrapped up at the site on Tuesday, the office of Switzerland’s attorney general said, with the surrounding area once again opened to hikers.

“Under coordination of the attorney general’s office, formal identification of the victims will now be carried out, and our investigations and clarifications will continue,” the authority said in a statement.