ZURICH (Reuters) - An investigation into the crash of a vintage plane in the Swiss Alps in August that killed 20 people has so far found no technical defects that caused the tragedy, the country’s transport safety agency said on Tuesday.

Three Austrians and 17 Swiss were aboard the Junker Ju-52 aircraft when it crashed on Aug. 4 in the mountainous canton of Grissons. The 1930s-era military aircraft was used by tourist flight operator JU-Air.

Some parts of the three-engined plane, including a portion of its wing, showed signs of corrosion and insufficient documentation of some modifications over the years may have posed risks, the Swiss Transportation Safety Investigation Board (STSIB) said in a 32-page interim report.

However, no concrete cause of the accident has been discovered so far, the STSIB said. It is not expected to complete its investigation before the middle of 2019.

“There were no indications through the date of publication of this interim report that pre-existing technical defects would have caused the accident,” the board said.

Founded in 1982, JU-Air offers sightseeing flights with German-made Ju-52 aircraft decommissioned by the Swiss Air Force.

After suspending services for two weeks following the crash, JU-Air resumed flights of its other two aircraft.

Accident investigators’ discovery of corrosion on the crashed plane has now prompted the temporary grounding of those planes, though JU-Air said the planes were already undergoing annual maintenance after the end of the summer flight season.

“JU-Air plans to re-start operations, as scheduled, in spring 2019,” the company said in a statement, adding it plans to add a third Ju-52 plane next year. It is awaiting a special permit to fly that aircraft from its current location, in the German city of Moenchengladbach, to Switzerland for additional maintenance work.