Authorities at the Golfech nuclear power plant in southwest France reported illegal drone sightings on Thursday night. This most recent sighting comes on the heels of previous illegal drone activity observed in the last month at seven other French nuclear power sites, largely during the nighttime hours.

The very first of these drones was spotted on October 5, flying over the Superphénix nuclear power plant in southeast France. French power giant EDF has filed a complaint with the police, but maintains that these flyovers are "without consequence for the safety or operation of these installations."

The Nuclear Safety Authority (ASN), which is tasked with ensuring that French nuclear sites are equipped to withstand accidental plane crashes or terror attacks, told AFP on Thursday that it "wouldn't discuss matters outside their field of expertise." Instead, it referred the matter to the French Air Force, which is responsible for monitoring the airspace above nuclear power sites. The ministry of defense declined to comment when contacted by VICE News.

It is illegal in France to fly over a power plant within three miles of the facility or at an altitude lower than 3,300 feet without a pre-authorization from the air force. EDF has confirmed that it was contacted by the air force following the illegal drone sightings.

Speaking Thursday on the French radio station France Info, minister of the interior Bernard Cazeneuve said, "Measures have been taken regarding this issue. There are investigations. The State has drone circumvention devices at its disposal." Cazeneuve, however, refused to qualify these devices.

The source of the drone activity remains unclear. Speculation initially turned to Greenpeace, whose activists have in the past staged interventions at nuclear sites throughout France, including one in 2012, during which the group flew over the Bugey plant in a powered hand-glider to highlight the lack of security.

However, Wednesday Greenpeace denied any involvement in the piloting of these mysterious drones.

Speaking to VICE News, Greenpeace's nuclear campaigner Yannick Rousselet denounced the lack of safety illuminated by these drone flyovers, saying, "[the authorities] let this happen because they did not have the capacity to react."

"Drones provide images whose resolution is only slightly better than those found online," Rousselet said, dismissing any idea of industrial espionage. "We could be looking at drone enthusiasts who gave themselves a challenge."

According to Rousselet, the drone activity bears the stamp of multiple perpetrators, mobilizing an impressive amount of resources.

"On October 19, for example, 4 [nuclear] sites, separated by hundreds of kilometers were flown over by drones," he explained.

Current data from the flights — time, distance, trajectory — is insufficient to determine whether the drones used were amateur or professional devices. A consumer level drone, costing only a few hundred dollars, could easily perform such tasks.

EDF's complaint concerns seven of its nuclear sites: Gravelines in the north, Cattenom in Moselle, the Blayais plant in Gironde, the Bugey plant in Ain, Chaux in the Ardennes and Nogent-sur-Seine in the Aube. Greenpeace activists maintain that an additional plant, the Fessenheim plant in Alsace, was also targeted.

These flyovers come at a time when France is in the throes of a tense public debate over the question of nuclear energy, with the Fessenheim power plant — the oldest nuclear site in France — at its heart. Deemed obsolete, many environmental groups have been calling for Fessenheim's immediate closure. Earlier this month, Emmanuelle Cosse, secretary general of the Europe Écologie-Les Verts green party, stated, "If Fessenheim isn't closed down, it means that France is unable to answer the question of nuclear safety. This plant is located on the biggest groundwater supply in Europe. Furthermore, Fessenheim suffers a breakdown every week."

Among the other nuclear sites that Greenpeace claims were targeted are the Pierrelatte plant, which is operated by French nuclear group Areva, and the Saclay Atomic Energy Commission (CEA), which is not a power plant but has several nuclear reactors.

Speaking to VICE News, the CEA expressed concern over several of its sites but declined to give further details, citing "the court case underway."