By Inti Landauro 

PARIS--Three Al Jazeera journalists who were detained by French police for flying a drone in a city park were freed Thursday morning, said a spokeswoman for the Paris prosecutor's office.

One of the trio--a U.S. citizen who was piloting the drone--will appear in court where he is likely to be fined a few hundred euros and have his drone confiscated, she said.

Flying drones over urban areas is banned in France and violation can theoretically lead to convictions of up to one year in jail and fines of up to EUR45,000 ($51,300), but French courts have so far imposed small fines and no jail terms, the spokeswoman said.

Paris has been puzzled by mystery drones that have periodically been spotted over the French capital's skyline. On two nights earlier in the week, the remote-controlled aircraft were seen over some city landmarks and high-security buildings, including the U.S. embassy. Last month, a drone was spotted flying briefly over the Élysée Palace, President François Hollande 's official residence.

In October last year, highly-secured nuclear power stations operated by power utility Electricité de France SA were also buzzed by mysterious drones. Remote-controlled drones--small, unmanned aerial vehicles--are available commercially and are popular in France.

Al Jazeera, a Qatar-based TV channel, said the journalists were actually filming a report on the mystery drones inside the Bois de Boulogne on the western edge of Paris when they were detained.

Two of the three men were cleared of any charges but the one handling the drone's remote control has agreed to appear in court where he will plead guilty, the spokeswoman said.

French police say they have no evidence to explain the purpose of the recent drone flights over the Paris skyline and don't know how many drones were involved or whether there was any coordination between the flights. The police said there is no evidence linking the Al Jazeera journalists to the night drone flights over the city.

While the investigation in Paris isn't related to antiterrorism, the flights come as the capital remains on high alert after terror attacks in January. Soldiers and armed police have maintained a heavy presence in front of state buildings and embassies around the capital.

Write to Inti Landauro at inti.landauro@wsj.com

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