A mother is suing the French government for €110,000 (£80,500; $120,000) for not preventing her son travelling to join Islamic State (Isis) militants in Syria.
The boy, identified as B, was 17 and a recent convert to Islam when he and three others left Nice in the south of France, boarding a plane to Turkey on December 27, 2013, and from there travelled to IS-held territory in Syria.
The boy's mother said she had a recent phone conversation with him, and he is still in Syria with the jihadist group.
She claims that she found out about his intentions from his friends in the housing estate on which they lived two days before he left, but was unable to prevent him going. She reported him missing 24 hours after he left.
"The police have committed a grave error...concerning this unaccompanied minor, who had a one-way ticket to Turkey with no baggage," the family's lawyer Samia Maktouf said.
She is seeking €110,000 for herself and her three children who remain in France.
The family said they were not interested primarily in the money, but wanted the French authorities to acknowledge their mistake in failing to prevent young people travelling to join IS.
The French Ministry of the Interior claims that because the boy was not under criminal investigation, there were no legal grounds for preventing him leaving the country.
"Turkey may be an entry route into Syria, but it is also a holiday destination," said the French government's counsel in court.
"Nothing in the behaviour of this adolescent showed that he was trying to leave the territory without the permission of his mother."
The mother was unable to attend the court session, after hearing that one of the companions of her son had been killed in Syria.
In April, the EU Justice Commissioner said that up to 6,000 Europeans had joined IS, including 1,400 French citizens.
French Prime Minister Manuel Valls warned in March that up to 10,000 Europeans could be waging jihad in Syria and Iraq by the end of the year. New laws which came into force this year give French authorities the power to bar those with suspected terrorist links travelling abroad.
French IS jihadists have released a number of propaganda videos urging supporters to commit terror attacks on French soil.