A Frenchman believed to have had links to a German al-Qaeda cell behind the 11 September attacks on the US has been remanded in custody in Paris.
Naamen Meziche, who is of Algerian descent, has been placed under formal investigation.
He was deported from Pakistan on Tuesday and faces charges of criminal conspiracy regarding a terrorist enterprise.
He was arrested in May last year in Pakistan close to the border with Iran.
Three other suspected French militants with whom he was detained were deported to France earlier this year.
Naamen Meziche was considered an associate of al-Qaeda commander Younis al-Mauritani, who was believed to have been ordered by Osama Bin Laden to plan attacks in Australia, Europe and the US.
"This is a big fish, at the historic heart of al-Qaeda," a French anti-terrorism official told AFP news agency.
However, analysts say there appears to be little evidence of his involvement in any attack and the case against him is unclear.
Born in Paris in 1970, Naamen Meziche travelled first to Afghanistan in the 1990s before moving to Germany.
He was considered close to the so-called Hamburg cell that planned the al-Qaeda attacks on New York and Washington in 2001.
He had reportedly recruited jihadists at a radical mosque in Hamburg that the authorities shut down in 2010 on suspicion of encouraging fanaticism.