Police in Paris yesterday detained three Al Jazeera journalists on charges of flying a drone without a license, marking the latest development in what's been a bizarre week for a city still reeling from last month's terrorist attacks. The arrests come amid growing anxiety surrounding unauthorized drones, or unmanned aerial vehicles (UAVs), which have been seen hovering above popular tourist sites and government buildings in recent weeks.

For two consecutive nights this week, small, non-military drones were spotted above some of Paris’ most famous landmarks, including the Eiffel Tower and the Invalides military museum, as well as the US embassy and major traffic junctions along the city's border. At least five drones were reported by witnesses and police each night, but their operators have yet to be identified. Paris police obtained video footage of some of the drones Tuesday night and are studying it as part of the ongoing investigation. At this point, it's unclear whether the drones were flown as a prank, a harmless flyover, or part of a coordinated or sinister operation, but French authorities are taking no chances.

"There's no need to worry, but we should be vigilant," government spokesman Stephane Le Foll told reporters Wednesday. "It's a subject being taken very seriously."

The journalists arrested Wednesday don’t appear to be linked to the drones seen overnight Monday and Tuesday. In a statement to The Guardian, an Al Jazeera spokesman said the journalists were actually filming a report on the drones seen earlier in the week. The journalist who operated the drone will appear in court next week, while the other two have been freed.

France has relatively stringent laws governing the use of civilian or commercial UAVs and is home to more than 1,200 commercial drone companies. (The US, by comparison, only recently adopted regulations for small commercial drones.) Operators can fly drones without a license below an altitude of 500 feet, but not above highly populated areas or sensitive government locations. In Paris, it’s illegal to fly any aircraft below 19,600 feet without a license, and flying UAVs at night is banned altogether. Offenders face a maximum one-year prison sentence and a €75,000 ($85,000) fine.

But these regulations haven’t stopped rogue operators from flying UAVs above sensitive sites. Between October and January, drones were reported flying above at least 13 nuclear power plants, and the operators’ identities remain unknown. In January, a drone was spotted above the Élysée Palace, home to President Francois Hollande.

Speaking to reporters this week, Le Foll stressed that mystery drones aren’t a uniquely French problem, pointing to similar incidents in Germany and the US, where a drone crash landed in front of the White House earlier this year. But Paris’ suspicious flyovers come at a time of heightened security concerns across France, following last month's flurry of jihadist attacks that began at the offices of the satirical magazine Charlie Hebdo. The government deployed thousands of police and military personnel to protect sensitive areas following the attacks, which left 17 dead, as part of a $480 million anti-terror program.

Following the nuclear plant incidents last year, France tasked its national research agency with developing new technologies to detect and intercept small commercial drones. That’s difficult to do at the moment, because smaller UAVs are hard to identify with radar systems, and techniques like radio frequency jamming could pose risks to other infrastructure.

Authorities say the drones spotted above the plants posed no immediate threat, but there are concerns that they could fall and injure civilians when flown above densely populated areas, like the UAVs spotted in Paris this week. In an interview this week with French daily Le Figaro, Christophe Naudin, an aerial security specialist, said the drones seen this week seemed to be flying simultaneously, suggesting a coordinated operation and, in his view, "excluding the possibility of a prank." Naudin hypothesizes that the operators are part of a "well-structured organization" looking to "test the intelligence capabilities" of the French government.

While the motives behind this week's spate of UAV sightings remain a mystery, it is clear that drones have become increasingly popular among amateur aviators. FNAC, the country's leading electronics retailer, described drones as "le hit" of the holiday shopping season last year. Christian Sanz, founder and CEO of the San Francisco-based commercial UAV maker Skycatch, told The Verge that we're likely to see even more rogue flyovers as drones become more mainstream, forcing governments and businesses to develop new technologies and stronger regulatory systems.

"Eventually, the consumer industry, in conjunction with lawmakers, will implement a system for detection of drones and their owners, and ensure a safe and secure technology for bringing unauthorized UAVs down," Sanz said in an email Wednesday. "Until then, you can expect to see more people pushing the boundaries, so long as they can't be detected."