Prosecutors say four long-time friends in their 30s, living in the same French city and communicating through a closed phone line, were planning a terror attack in France as early as next week on orders from Islamic State.
The four arrested on Sunday in the eastern city of Strasbourg plotted to carry out an attack on December 1, but investigators have not yet determined "the specific chosen target among all those considered by the group," prosecutor Francois Molins said.
A fifth suspect was arrested in the southern city of Marseille at the same time as the Strasbourg suspects.
Molins said that suspect was not in direct touch with the other four, but was "given guidance remotely" from the same IS member.
The prosecutor didn't name any targets, but security was tightened this week at the Paris headquarters of France's criminal investigations police, reportedly among the locations studied.
Disneyland Paris was reportedly another target the group considered.
The night they were arrested, two of the Strasbourg group had just downloaded the Periscope application, which allows people to stream live on the internet with a mobile phone.
The app activity suggests they were preparing an "imminent" attack, Molins said.
The four Strasbourg suspects also were in possession of guns and ammunition, he said.
Among the weapons seized during home searches were two handguns, two automatic rifles, several cartridge clips and dozens of cartridges.
Investigators also found instructions for a money transfer, GPS coordinates and detailed explanations for obtaining more weapons on one suspect's USB stick.
All five men had a "clear will to find and to identify targets to commit an act in the very short term," Molins said.
Investigators also recovered a notebook that contained 12 pages of writing that referred to an armed jihad, death in martyrdom and Abu Bakr al-Baghdadi, the Islamic State leader.
The four arrested in Strasbourg were two French citizens, both age 37; a 36-year-old Franco-Tunisian; and a 35-year-old Franco-Moroccan.
The man arrested in Marseille was a 26-year-old Moroccan.
Two of the Strasbourg suspects traveled to the Turkish-Syrian border via Cyprus in March 2015, then prosecutor said.
The Marseille suspect left Morocco in 2013 and made multiple trips across Europe with fake ID documents.
In 2015, the Turkish authorities prevented him from entering Turkey, he said.
