Authorities in France on Thursday announced the launch of an investigation into a series of mysterious drones that have been sighted flying over a number of the nation's nuclear power facilities.

Unmanned aircraft have been documented flying over seven separate facilities operated by the state-owned nuclear power company EDF between Oct. 5 and Oct. 20, according to company officials.

"There's a judicial investigation under way, measures are being taken to know what these drones are and neutralize them," said Interior Minister Bernard Cazeneuve on a French news station on Thursday.

Reuters reports:

EDF named the plants over which drones had been spotted as Creys-Malville and Bugey in the southeast, Blayais in the soutwest, Cattenom and Chooz in the northeast, Gravelines in the north and Nogent-sur-Seine, the closest plant to Paris. The unmanned aircraft were spotted late in the evening, at night or very early in the morning, EDF said. It is prohibited to fly less than 1,000 meters above nuclear plants and within a 5 kilometer radius. Each plant has filed a formal complaint with the police against the anonymous people behind the drone flights.

Though some fingers first pointed at the anti-nuclear group Greenpeace, which has heavily criticized France's reliance on nuclear energy and staged provocative protests at some of the nation's plants in order to highlight their vulnerability to sabotage or attack, the group on Thursday said they have nothing to do with the recent drone activity.

"For all its actions, Greenpeace always acts openly and claim responsibility," Yannick Rousselet, head of Greenpeace's anti-nuclear campaign, said in a statement. However, he added, "What is happening is very worrying."

BBC reports:

Under French law, no aircraft is allowed within 5km or 1,000m of a nuclear plant and the air force is responsible for the protection of all such sites. France is 75% reliant on nuclear power for its electricity and has 58 reactors operated by EDF at 19 sites. According to the company, the first drone flew over its Creys-Malville plant at Isere in south-east France, 50km (31 miles) east of Lyon. Most of the flights took place between 13 and 20 October and either at night or early in the morning, the company says.

Reporting at Quartz adds that "[at] this stage, it isn’t clear whether the drone flights are the work of hobbyists, another green group, or something more sinister. Nuclear energy generates nearly three-quarters of France’s electricity, the largest share in the world. The government has pledged to cap its current nuclear capacity, boosting renewables and other energy sources so that only half of the country’s power comes from nukes by 2025. If fears over the safety of France’s extensive reactor network rise, it might hasten the shift."