The investigators were trying to determine whether or not the Tunisian attacker, Mohamed Lahouaiej Bouhlel, had received logistical support from them, BFMTV news reported.
Prosecutor Francois Molins said Lahouaiej Bouhlel had prepared the attack for months with help of accomplices.
At least four men and a woman, believed to have been involved in the preparation of the July 14 attack which also left more than 300 people injured, have already been placed under investigation.
The Islamic State has claimed responsibility for the attack, saying one of its "soldiers" acted in response to the coalition attacking the group.
France has been put under high security alert after recent attacks in the country claimed by the IS group.
This website uses cookies.