Seven men suspected of fighting in Syria have been placed under formal investigation following their arrest in Strasbourg, said a judicial source on Saturday, weeks after France unveiled policies to prevent French Muslims from becoming radicalized. They were suspected of travelling to Syria late last year to join the uprising against Syrian President Bashar al-Assad. European governments calculate several thousand of their citizens have gone to Syria since the war began three years ago and are worried about the security risk posed by youths radicalized by the conflict. France, which estimates around 285 of its citizens are currently fighting in Syria among the rebels' ranks, last month launched a plan to prevent its citizens joining the civil war and becoming a threat to their home country.