Paris attacks suspect Salah Abdeslam has appeared before a French judge hours after his extradition from Belgium.
His lawyer said the 26-year-old Belgian-born French national is being formally investigated for murder and attempted murder of a terrorist nature.
Salah Abdeslam is thought to have played a key role in planning the Paris attacks and transporting the attackers.
He was arrested in an 18 March raid in Brussels after four months on the run.
The coordinated attacks carried out by Islamic State in Paris on 13 November last year claimed 130 lives and left dozens more severely wounded.
Belgium's federal prosecutor said Salah Abdeslam had been "surrendered to the French authorities this morning (in execution of the European Arrest Warrant issued by France on 19 March 2016)".
Until his next appearance in court on 20 May he will be kept in custody at Fleury-Merogis, Europe's biggest jail a short distance south of the capital where two other Islamist militants served sentences before taking part in the January 2015 attacks on Paris.
Salah Abdeslam was indicted by Belgian authorities last week over a shoot-out in the Forest area of Brussels in which four police were wounded, three days before he was arrested.
He left Beveren prison near Antwerp on Wednesday under heavy French security as French criminal lawyer Frank Berton announced that he would be taking on his defence.
Mr Berton said they had had a two-and-a-half-hour meeting at the end of last week.
He told the French news channel BFMTV he hoped the man would be judged "for what he has done and not what he has not done... not for what he represents because he is the last survivor".
All the other key Paris attackers are believed to be dead.
Mr Berton said Mr Abdeslam was "falling apart because of the drama that unfolded in France" and that he wanted to explain his actions.
He was aware of the risks in representing Mr Abdeslam, but insisted his client had the right to a fair trial.
Mr Abdeslam's Belgian lawyer, Sven Mary, has spoken of the dangers he faced in representing Mr Abdeslam - telling France's Liberation website he was threatened, assaulted physically and verbally in his office, and that on occasion police had escorted his daughters to school.
But Mr Mary also spoke disparagingly of his client, describing him as having the "intelligence of an empty ashtray".