A plane landing in the northern Spanish city of Bilbao was forced to avoid three drones while landing on Saturday.
The pilot of the Lufthansa aeroplane told air traffic control that he was forced to avoid three drones that were flying at an altitude of 900 metres when the plane was already in protected airspace and starting its descent.
The Airbus 320, flying from Frankfurt to Bilbao, was forced to perform an "evasive manoeuvre" to avoid the unmanned aerial vehicles.
While the size and weight of the drone is unclear, only a professional brand could have reached such a high altitude.
Basque police received notice of the incident from the airport and deployed a helicopter to patrol the skies around the airport in search of the drones but found no trace of them.
Patrols on the ground also had no luck in finding the drone owners, police sources told Spanish news agency, Efe.
It is illegal to fly drones inside the restricted air space surrounding Spanish airports, as well as flying drones higher than an altitude of 120 metres.
The case is currently being investigated by the Spanish Agency for Aerial Navigation.
It is not the first case of drones interfering with planes. In March an Air France plane had a very near miss with a drone when it was coming in to land at Paris Charles de Gaulle airport.