French authorities arrested three Al Jazeera journalists Wednesday for flying a drone in western Paris. Police have been investigating a swarm of unidentified drone sightings over the French capital in recent nights, increasing tension in a country already on high alert for terrorist threats.

Police took the trio into custody after spotting a drone flying in the city's Bois de Boulogne park, a spokeswoman for Paris prosecutors told the Associated Press. She said it wasn't clear what the reporters were attempting to do. Prosecutors said there was "no relationship for the moment" between the arrests and the separate drone flights over the city on Monday and Tuesday nights.

Flying drones without license over Paris carries a maximum one-year prison sentence and an $85,000 fine.

The reporters, ages 70, 54 and 36, were "filming a report on the city's recent mysterious drones," Al Jazeera said in a comment from its base in Doha, Qatar. The international broadcaster said it "will comment further when more information is available."

A judicial source said of the three people arrested: "The first was piloting the drone, the second was filming and the third was watching," Agence France-Presse reported.

Earlier this week, unidentified flying objects were spotted near Paris landmarks, including the Gare de l'Est train station, the Paris Opera, the Tuileries Garden, the Eiffel Tower and the Montparnasse Tower, AFP noted. Drones also were spotted near the U.S. Embassy in Paris.

The mysterious activity is elevating security concerns sparked by the deadly terrorist attacks in Paris last month. The Jan. 7 shooting at the offices of satirical magazine Charlie Hebdo left 12 people dead and shifted the country to a state of high security.

French authorities have said the recent drones pose no threat, aside from potentially falling on someone. But officials are trying to contain and counteract the vehicles, which some fear could eventually be used to spy on technology or fire weapons, the AP said.