When mysterious drones began appearing over oil rigs and wind farms off Norway’s coast about three years ago, officials were not certain where they came from.

But "we knew what they were doing,” Stale Ulriksen, a researcher at the Royal Norwegian Naval Academy, said in a recent interview. "Some of it was espionage, where they are charting a lot of things. Some of it, I think, was positioning in case of a war or a deep crisis.”

The drones were suspected of being launched from Russian-controlled ships in the North Sea, Ulriksen said, including some ships that were near underwater energy pipelines. Norway could not do much to stop them, he added, given that they were flying over international waters.