There is no evidence that foreign powers were behind drones sighted at Swedish nuclear plants in January, the prosecutor leading the investigation has told publicly funded Swedish Radio.
Police in Sweden deployed patrols and helicopters in mid-January to the Forsmark nuclear plant to hunt for a drone seen flying over the site but were unable to catch the unmanned vehicle.
Drones were also observed at Sweden's two other plants, Oskarshamn and Ringhals.
Forsmark and Ringhals are run by state-owned Vattenfall while Oskarshamn is operated by German energy company Uniper SE.
The three nuclear plants account for about 30 per cent of Sweden's total power production.
Prosecutor Hans Ihrman told Swedish Radio there was no evidence that foreign powers were behind the drones and said it was unclear what had actually been observed.
The incidents occurred around the same time as Sweden's military started patrolling the main town on the Baltic Sea island of Gotland amid increased tensions between NATO and Russia.