The four French journalists safely returned to France last month after being held as hostages in Syria for 10 months reported the shocking news that some of their captors spoke perfect French. The men were never allowed to see their hooded guards’ faces, and were told they would be killed if they did. The idea that French citizens might have been among the hostages’ captors has shaken the French public, and the government moved quickly to announce a plan to prevent its citizens from joining extremist groups in Syria.

Some of the measures will be difficult to put into effect, and some raise legal and civil liberties issues, but there is no question that the government has to act. The French Foreign Ministry estimates that 500 to 700 citizens have left the country to join radical groups fighting in Syria, like the Islamist State of Iraq and the Levant, which is suspected of kidnapping the French journalists. Many are teenagers or young adults who have become radicalized by visiting websites that promote jihad in Syria as holy war.

The government will take aim at extremist Islamic websites; reinstate a requirement that minors have parental permission to leave French territory; and register profiles of people deemed to pose a risk with French agencies and with the Schengen Information System, the European database.

On April 30, the French, British, Belgians and Germans met in London to discuss how best to tackle this growing, pan-European problem. France is cool to Britain’s plans to encourage British Muslims who want to help Syrians to do so through charitable groups, and to enlist women to work with the police to identify potential jihadists.