According to a report by Bild newspaper, prosecutors believe that the drones were Russian built Orlan-10 drones, which have a range of over 300 miles and could have been deployed from civilian shipping in the North Sea.
In May, Jens Stoltenberg, Nato’s secretary general, warned that Russia had increased its subterfuge efforts, saying that “we have seen several arrests across the alliance of people accused of arson and sabotage”.
Incidents being treated as suspicion include a fire at a shopping centre in Poland’s capital Warsaw and suspected arson at an Ikea store in Lithuania.
In Germany, several suspected Russian spies have been detained in recent months.
In April, two men were arrested on suspicion of planning to set off an explosion at a military or industrial site in an attempt to destabilise Germany’s support for the Ukrainian military.