Paris (AFP) - Paris police were hunting on Tuesday for clues as to who carried out late night drone flights over at least five landmarks in the French capital, including the Eiffel Tower.
The mysterious flyovers came after a series of drone spottings at French nuclear plants, and more recently over the presidential palace.
The fresh sightings also come at a time of heightened security in Paris following last month's jihadist attacks that left 17 people dead.
The first drone sighting was near the US embassy in the French capital just after midnight, prompting police to follow the unmanned aircraft which continued on towards the Invalides military museum.
But they soon lost sight of the drone and later in the night, four other pilotless aircraft were spotted at several Paris landmarks including the Eiffel Tower, the Place de la Concorde and the Montparnasse tower, the tallest skyscraper in the French capital.
"It could be a coordinated action but we don't know for now," said one source, who asked not to be identified.
"We did everything to try and catch the operators but they were not found," another source close to the case said.
A police source, who also wished to remain anonymous, said it was not clear how many individual drones were involved in the flyovers which saw security forces mobilised for six hours trying to track down the operators.
In October and November, around 20 of the unidentified aircraft flew over French nuclear plants and their operators were never found.
Also in October a 24-year-old Israeli tourist spent a night in jail and was slapped with a 400-euro ($500) fine for flying a drone above the Notre Dame cathedral.
Then on January 20, a pilotless aircraft briefly went over the presidential palace in Paris, just a week after three days of jihadist attacks in Paris put the country on heightened alert.
- Threat or no threat? -
And at the end of January, small drones were spotted near a bay in Brittany that houses four nuclear submarines -- one of the most protected sites in the country.
French law bans small civilian drones from areas such as nuclear facilities, which are protected by a no-fly zone that spans a 2.5-kilometre (1.6-mile) radius and a height of 1,000 metres.
Experts say that the small unmanned craft would not pose a threat if they crashed into a hardened nuclear facility, whose reactors are built to withstand the impact of mid-sized commercial planes.
But the country has nevertheless launched a one-million-euro ($1.1-million) programme aimed at developing ways of detecting and intercepting them.
Drones come in all shapes and sizes, and have a variety of uses, from widely reported military applications to surveillance, filmmaking, sports, disaster relief and scientific research.
"Is it a game? Scouting for future operations? The investigation will show us," said a Parisian police chief.
"It is clearly mysterious and even if it doesn't appear terror-related, we are taking this case very seriously," he added.
A judicial source, speaking on condition of anonymity, said a probe had been launched by Paris prosecutors into "flying an aircraft in a forbidden zone."
And police officials said a dedicated team of air transport officers had been formed to tackle the incident, and that witnesses were being questioned.
"There are many eyewitness accounts and investigators must sift through actual descriptions and others to determine the exact path of these drones," an investigator said.
The most basic unmanned aircraft are radio-controlled by someone nearby, but other more sophisticated models can be pre-programmed, and these are widely available for just 350 to 400 euros ($440 to $500).