NATO's air base outside Madrid, which monitors the skies of southern European countries, has continued the investigation into who launched the military drone that crashed in Zagreb last week, but so far it remains unknown whether it was an error, an act of sabotage or an attack.

Croatia’s Air Force Commander Michael Krizanec, Air Observation Battalion Commander Kresimir Tafra, Security and Intelligence Agency (SOA) Director Danijel Markic and SOA’s Chief of International Cooperation Robert Filipovic-Persin joined the investigators at Torrejón de Ardoz Air Base on Wednesday.

They arrived in the morning with Prime Minister Andrej Plenkovic. Plenkovic met with the Base Commander, Fernando de la Cruz, and was shown how the air monitoring system worked.

Reporters were not allowed into the base. A member of the Croatian delegation told Croatian reporters that Plenkovic’s visit took about an hour, during which he discussed the drone incident and toured the base.

After the visit, Plenkovic travelled to Madrid to meet with Spanish Prime Minister Pedro Sánchez.