Several unidentified drones flew close to the Gösgen nuclear plant on Sunday night, and police in Solothurn are still trying to work out who was behind them.
A local resident spotted the aircraft around 10.30pm, and alerted patrols, who confirmed multiple drones over the exclusion zone that surrounds the site.
Investigators are working with neighbouring cantonal forces, the Federal Office of Civil Aviation and Fedpol to trace the route and identify the operators.
The plant's operator says security around the perimeter was tightened once the alert came in, and that nuclear operations were never compromised.
Drone flights are banned outright over the plant, with offenders facing up to five years in prison in the most serious cases.
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