Bulgarian authorities have disclosed that they are probing the in-flight loss of parts from an Air Serbia ATR 72-200 in the vicinity of Sofia.
The aircraft (YU-ALO) had departed Sofia for Belgrade on 26 November.
French investigation authority BEA, citing Bulgarian counterparts, says the turboprop's crew opted to return to Sofia owing to "vibrations and noise" during the climb.
The aircraft landed without further incident at Sofia some 15min after departure.
While the details of the event have yet to be fully disclosed, BEA says that the incident involved "loss of part of the aircraft structure" during flight.
Flight Fleets Analyzer lists YU-ALO as a 27-year old airframe originally delivered to Air Serbia predecessor Jat Airways.