Two suspected al-Qaeda militants have been arrested at France's main airport in recent days, French officials say.
More than 3,000 people were killed in the 11 September attacks
Karim Mehdi, a Moroccan national, was taken into custody at Charles de Gaulle airport near Paris on Sunday.
Investigators believe he is linked to al-Qaeda militants based in Germany who planned the attacks on New York and Washington.
The second suspect, Christian Ganczarski of Germany, was arrested on Monday.
He is due to be interviewed by a judge in the coming days.
France's top anti-terrorist magistrate, Jean-Louis Bruguiere, has already put Mr Mehdi under official investigation - one step short of pressing charges - for "participation in an association of criminals linked to terrorist activities".
Officials say Mr Mehdi, 34, had arrived from Germany and was travelling to the French island of La Reunion, in the Indian Ocean, when he was arrested.
The sources say he was planning an attack a tourist resort there.
Mr Ganczarski, 36, is described as an associate of Mr Mehdi.
Mounir al-Motassadek is the only man convicted for the attacks so far
Investigators believe Mr Ganczarski is linked to the April 2002 suicide attack against a synagogue in Djerba, Tunisia, which killed 21 people - including 14 German tourists.
He was briefly arrested by German police after the attack, but no charges were brought against him.
Several of the 19 men who hijacked four planes and deliberately crashed them in Washington and New York on 11 September 2001 once lived in Hamburg.
The only person to have been tried and convicted for any role in the attacks was also linked to Hamburg.
Mounir al-Motassadek, a Moroccan, was jailed for his part in the attacks by a court in the city where he was an acquaintance of Atta.