The two boys, known as Y and A, are 15 and 16 years old respectively.
They could be charged with "participation in a criminal conspiracy in relation to a terrorist enterprise", although pressing charges is rare in cases involving minors and the headmaster of their school in Toulouse has appealed for clemency.
They were brought back to France on 26 and 27 January, having gone to Turkey on 6 January with the intention of joining Islamist fighters in Syria.
The younger boy managed to cross the border but not to get to the front line.
Investigators say that there is no evidence that they were recruited by a network in France but want to find out if they were helped in Turkey by "facilitators" who had come from France.
The younger boy has no record of trouble with the police but the older one was known to them in connection with minor offences and is related to several Islamist hardliners.
A 15-year-old girl, Nora, from Avignon who has been missing for a week has left to fight in Syria, according to a local paper, Vaucluse Matin, which says that she declared her intention to fight "holy war" on social media and telephoned her family from the Turkey-Syria border on Thursday.