Paris prosecutor Francois Molins said on Friday that the youngest of three suspects in custody -- aged only 17 -- was given the
order when it became clear he would not be able to leave the country to wage jihad in Syria because he was under surveillance.
The teenager and two other young Islamic radicals are all set to appear before a judge on Friday when they are expected to be charged with terrorism offences. The three young men, including a former naval signalman, were arrested earlier this week and have been questioned by France's intelligence services.
They are accused of plotting to kidnap and decapitate a member of the armed forces at a military base.
They were transferred to Paris on Thursday evening and will appear before a judge in the capital on Friday, after which they are expected to be charged with conspiracy to commit terrorist acts.
Interior Minister Bernard Cazeneuve said security forces had staged dawn raids on Monday and arrested four people, aged between 16 and 23, who were "planning to commit a terrorist act" at a French military installation.
The youngest was quickly released but the other three are suspected of planning to kidnap and behead a member of the military on film, possibly on December 31 when the facility was thinly staffed.
(Fort Bear, where the three allegedly planned to kidnap and behead an army officer. Photo: AFP)
The oldest of the group served as a navy signalman at the base around the southern town of Collioure, which is also used for training by elite commando forces.
Identified only as Djebril, he was discharged from the navy in January 2014 for back problems, said a source close to the investigation, and the target is thought to have been his former boss.
The other key plotter was just 17, and was already being closely watched by authorities due to his activities on social media and connections to French jihadists in prison.
All three had been planning to travel to jihadist-controlled areas of Syria, the security source said, but the 17-year-old's mother became concerned about his radicalisation and contacted the authorities.
He was interviewed by counter-terrorism officials and was aware he was under surveillance.
"They claim to be part of Daesh," said a source close to the investigation, using an alternative Arabic acronym to refer to the jihadist Islamic State group that controls swathes of Syria and Iraq.