Prosecutors in France are investigating the possibility that French citizens are being recruited into the ranks of the insurgency in Iraq, officials say.
Iraqi insurgents have been joined by fighters from abroad
Suspicions were aroused after probes into domestic Islamic extremists, a judicial official said.
However, officials said the number of people suspected of signing up for a jihad, or holy war, in Iraq is small.
Judges heading the investigation can question suspects and demand information from other authorities.
The probe will be conducted by top anti-terror magistrates, Jean-Louis Bruguiere and Jean-Francois Ricard.
At the centre of the inquiry are around 10 people who may have travelled from France to Syria to join the insurgency in Iraq, according to an anonymous judicial source quoted by the French news agency AFP.
The arrest of several suspected extremists at a mosque in suburban Paris on 15 June fuelled the suspicions, the source said.
According to the source, a mobile phone belonging to one of the suspects contained a text message, originating from Iraq, which read: "Group has arrived. I will contact you if I need help."
Separately, police believe a French national of North African origin - whose body was discovered in July in the Iraqi militant stronghold of Falluja - may have gone there to fight against US-led forces, according to French newspaper, Le Figaro.
Last year, investigators in Italy concluded their country was being used a staging post by militants on their way to Iraq.
In May this year, Italian police arrested several people on suspicion of plotting attacks against western and government targets in Iraq.