The DST, France's domestic security agency, conducted the roundup of suspects in the suburbs of Paris on orders of two French anti-terrorism judges, Jean-Louis Bruguiere and Jean-Francois Ricard.
One person was detained at Charles de Gaulle airport as he tried to leave the country, Paris prosecutor Yves Bot said.
The operation was part of an investigation into, that killed 33 bystanders and 12 bombers on May 16, 2003, authorities said. Paris prosecutors opened an investigation three days after the attacks because three of the victims were French.
Those detained for questioning are suspected of belonging to the Moroccan Islamic Combatant Group, Bot said. The organization, which has alleged links to al Qaeda, has been blamed by the Spanish government in the.
However, Bot said authorities do not have evidence linking the suspects in France to the Madrid bombings.
"These investigations are the culmination of a long probe … into the networks linked to the Moroccan Islamic Combatant Group," Bot said in a statement.
The DST conducted the operation as part of an investigation involving other foreign intelligence services, the Interior Ministry said in a statement.
Agents raided eight locations in the early morning in search of suspects "directly implicated" in the Casablanca attacks or those responsible for planting bombs, police sources said.
The identities of the suspects were not released. Under French law, they can be held in detention for up to 96 hours without being formally placed under investigation.
Fifteen suspects are in custody in the Madrid attacks. Six have been charged with mass murder and nine with collaborating with or belonging to a terrorist organization. Eleven of the 15 charged are Moroccan.
Two weeks ago, French anti-terrorist police detained three suspects in connection with an investigation into a mysterious group's threats to bomb French railways.
The suspects, two men and a woman, were taken into custody Thursday in Paris and the suburban Val-de-Marne region, police said.
An obscure group that calls itself AZF has threatened to detonate bombs at French railway targets unless it is paid millions of dollars.
The group issued a cryptic letter threatening an attack to surpass the Madrid bombings, but also announced it was suspending its operations so it can perfect them.
A bomb was found two weeks ago half-buried on a train track near the town of Troyes, some 100 miles southeast of Paris, triggering a massive inspection of France's rail network.
It was the second bomb discovered hidden under tracks in just over a month — and the second inspection of thousands of miles of track. The first bomb, found in February, was claimed by AZF. There was no claim of responsibility for the second bomb.
The Moroccan Islamic Combatant Group is a forerunner to the Salafia Jihadia and is regarded as the first radical jihad movement in Morocco.
According to the U.S. State Department, the Moroccan Islamic Combatant Group formed in the 1990s from fighters who had trained in camps in Afghanistan and has as its goals "establishing an Islamic state in Morocco and supporting al Qaeda's jihad against the West."