The brother-in-law of one of the jihadists who attacked French satirical magazine Charlie Hebdo last year faces terrorism charges for allegedly trying to join so-called Islamic State fighters in Syria, a judicial source said.
French student Mourad Hamyd, 20, was charged by an anti-terrorism judge in Paris with "associating with terrorists" and held in custody.
Mr Hamyd, whose sister Izzana was married to Charlie Hebdo gunman Cherif Kouachi, had been sent back to France yesterday from Bulgaria where he had tried to cross the border into Turkey in late July.
But Turkish officials handed him over to Bulgarian border authorities.
France requested Mr Hamyd's extradition on 29 July, accusing him of "conspiring to prepare acts of terrorism".
French prosecutors said that the route taken by Mr Hamyd - by train through Austria, Hungary, Serbia and Bulgaria - corresponded to the one traditionally taken by jihadist fighters wanting to join IS in Syria or Iraq.
Mr Hamyd told Bulgarian authorities that he was only a tourist and had "no ties" to IS, but nevertheless he agreed to be returned to France.
Al-Qaeda-linked Cherif Kouachi together with his brother Said, killed 12 people at Charlie Hebdo's Paris headquarters on 7 January 2015 in the first of a series of attacks that have hit France over the past year and a half.
Mr Hamyd first came into the spotlight in the immediate aftermath of the Charlie Hebdo attack, when he was wrongly identified on social media as being among the killers.
He was taken in for questioning and later freed.