I found the enquiry report
https://www.sust.admin.ch/inhalte/AV-berichte/2375.pdf.
In French, but basically says the engine failed to produce enough power during or soon after takeoff. The instructor took over from the pilot but could not establish a safe climb. The injector regulator was blocked. Some criticism was levelled in that the regulator had not been revised since 1986. I am no expert but I gather that after the accident to the aircraft in 2016 it should have been revised. There is a reference to the fact that the accident might have been suvivable for the instructor if he had fastened his shoulder harness (as had the other two occupants, who survived).
Laurence