Fourteen of the victims were German tourists, and responsibility was claimed by a group linked to Osama bin Laden's al Qaeda organization that also took credit for the 1998 bombings of U.S. embassies in Kenya and Tanzania.
French authorities have said they believe the attack was carried out by a Tunisian, Nizar Naouar, and an unidentified accomplice who also lived in the North African country. Officials said Naouar is believed to have died in the explosion, but they have not said what happened to his alleged helper.
One of those detained was identified as Walid Naouar, the brother of Nizar Naouar who believed to have been driving the gas truck that exploded outside the synagogue on Tunisia's resort island of Djerba.
Naouar's parents and three people close to the family were among the others detained, Lyon prosecutor Christian Hassensrat said. Under France's tough new anti-terrorism law, authorities can hold the suspects up to four days without charging them.
The suspects were detained near Lyon in southern France and were being questioned by agents from France's DST counterintelligence service who were acting on orders from France's top anti-terrorism judge, Jean-Louis Bruguiere.
Interior Minister Nicolas Sarkozy said in a statement that police found documents in the Naouar home in Saint-Priest, near Lyon, that relate directly to the attack.
Police said it was too soon to determine if the family played any role in the attack but their arrests had been planned for some time because of their "peripheral interest" to investigators.
At the time of the attack, members of the Naouar family said they were shocked at their relative's alleged connection to the explosion.
"We have nothing to hide, we're just looking for the truth," Walid Naouar told The Associated Press in a telephone interview a week after the attack.
Walid was later detained by French border police at the Lyon airport for lacking proper French papers. He was threatened with expulsion to Tunisia, where the Naouar family comes from, but the process was halted because of a technicality.
The 2,000-year-old Ghriba synagogue, the oldest in Africa, was full of tourists at the time of the April 11 blast, about 375 miles south of Tunis. The synagogue is the symbolic hub of the Muslim North African nation's approximately 2,000-strong Jewish community.
Last month, Osama bin Laden's No. 2, Ayman al-Zawahri, said U.S. allies needed to get out of the Muslim world, adding that warnings had already been sent to Germany and France. A month after the synagogue blast in Tunisia, an attack on a bus in Pakistan killed 11 French engineers. That attack has also been linked to al Qaeda.
The Tunisian government initially maintained the explosion was an accident, but later acknowledged the blast was "a premeditated criminal act."