Two writers face probe after allegedly asking for money in exchange for not publishing 'damaging' book about King Mohammed VI.
The arrests, which are not thought to be linked to the night-time flights over Paris, came after police saw drone flying in the west of the city.
Broadcasters who employed the reporters, who were killed doing their job, launch campaign for end to impunity for crimes against journalists.
Expert says the Islamic State in Iraq and the Levant (ISIL) jihadist group in Syria lacks funding 'so they have to use their hostages'.
Journalist Didier Francois said the four were chained to each other and kept in basements without natural light; ministers deny ransom was paid.
Nicolas Hénin says release of him and three other French journalists was a 'real surprise' and that he is 'filled with joy' but exhausted.
The four, held captive for almost a year, were found by Turkish soldiers on the border with Syria blindfolded and with their hands bound.
French foreign minister said the two radio journalists were shot in a 'cold blooded' assasination, while it was also reported their throats were cut.
The kidnapping of Nicolas Henin of Le Point magazine and TV journalist Pierre Torres on June 22 had not previously been made public.
Didier François, a radio reporter, and photographer Edouard Elias were kidnapped near the northern Syrian city of Aleppo in June.
Bernard Bernard Squarcini, ex-head of the DCRI, is to face trial for seizing phone records of a journalist investigating Bettencourt affair.
President Francois Hollande said the journalists, French radio station Europe 1's Didier Francois and Edouard Elias, should be freed immediately.
Two French journalists who were smuggled out of Syria after being trapped for nine days in the war-torn country's town have arrived back in France.