The suspects appeared before an investigating judge on Friday, who didn't take long before putting them under formal investigation. The men will remain in custody until their trial, whose date is still unkown.
They belong to a group of fifteen members from the residential town of Meinau in Strasbourg, and were arrested during a dawn raid on Tuesday.
The men, two brothers, and several previous offenders, are suspected of going to Syria to fight with radical Islamist groups against President Bashar al-Assad.
It's thought they lied to their parents that they were going on holiday to Dubai. Their flight provoked a massive mobilisation at the beginning of the year against the brain-washing of young people by Al-Qaida propaganda on the Internet.
Another man Mourad Fares, aged 29, is also being investigated for recruiting young men and women on social networks, according to a judicial source.
Around 800 people in France have so far gone to Syria to join the ranks of Al-Qaida affiliated groups in Damascus. French police are profiling dozens of potential recruits, as part of its new anti-jihad campaign.