France has placed a German citizen under investigation over last year's bomb attack on a Tunisian synagogue.
Many of the dead at the historic site were German tourists
Christian Ganczarski was arrested on Monday at Charles de Gaulle airport near Paris and is believed by investigators to have links to the Islamic militant network al-Qaeda.
A second man, Moroccan national Karim Mehdi, was taken into custody at the same airport on Sunday on suspicion of links to militants behind the 11 September attacks on New York and Washington.
Mr Mehdi, 34, has already been placed under investigation by the French authorities - the final step before criminal charges can be brought.
Intelligence agencies have long suspected Mr Ganczarski, a 36-year-old Polish-born Muslim convert, of al-Qaeda connections, the BBC's David Bamford reports.
Mr Ganczarski was detained twice before - in Germany and Saudi Arabia - but each time was released through lack of evidence.
He has admitted knowing the suspected suicide bomber in Tunisia though denied involvement in the attack.
French officials say he also admits knowing Mr Mehdi who was detained as he was about to board a plane for the French territory of Reunion in the Indian Ocean.
Investigators are working to a theory that he may have been involved in plans to attack tourist centres, an allegation denied by Mr Mehdi.
The April 2002 suicide attack against the synagogue in Djerba, Tunisia, killed 21 people, including 14 German tourists.