Mysterious drones have been spotted flying over nuclear power plants in France in the past three weeks, leading to a legal complaint, an investigation and fears of a potential attack. Officials recorded about 15 unauthorized drone flights over at least seven facilities in October, the Associated Press reported. The drones did not impact the facilities' security or operation, but the Telegraph reported they were big enough to transport explosives. 

It's illegal to fly within three miles of or less than 1,100 yards above a nuclear facility, according to the Washington Times. The drones were commercially available and easy to operate, the Huffington Post reported. The flights, though miles apart, followed a similar pattern.

The Électricité de France told the Guardian the drones were seen Oct. 5 and between Oct. 13 and Oct. 20, usually at night or in the morning. On Oct. 19, there were four flights -- one in east France, two in the north and one in the north-central region. The latest incident was Friday.

"Drone overflights are currently being carried out in a repeated and simultaneous manner over certain nuclear sites in our country," the prime minister's general secretariat for defense and national security said in a statement to the AP. "The objective apparently sought by this type of organized provocation is to disrupt the chain of surveillance and protection at these sites."

They were not shot down because they were not a threat, a spokesman told the Guardian.  The nuclear power plants were built to withstand events like earthquakes and plane crashes.

The drones' source was unknown. The Associated Press reported there were no leads, but the people involved could be fined or spend time in prison. Greenpeace, an organization of activists known for protesting nuclear power, told officials they were not responsible. There was concern it could be a large-scale operation, according to BBC News.

Nuclear power accounts for two-thirds of France's electricity. French officials minimized the risk in interviews but said they were working to identify and neutralize the drones.