Croatia Control said on Friday the drone which entered Croatia's airspace last night and crashed in Zagreb did not have a transponder, which is why Croatia Control did not detect it on secondary radars used to monitor aircraft.

In Croatia, in order to monitor the controlled airspace, all aircraft entering Croatia's airspace must use a transponder, Croatia Control said, adding that planes without transponders are not visible to flight controllers.

A subsequent check of data from the primary radar on the Pleso location revealed that a radar blip and a flight path, corresponding to the time and location of the drone's crash, were detected.