BETHONCOURT, France — French intelligence officials got in touch with the parents of a shy, 15-year-old Muslim girl in this depressed town in eastern France last May to convey some shocking news: Their daughter had become a frequent visitor to websites preaching jihad.

The parents asked the French authorities to take steps to block their child from leaving the country and took possession of her passport, according to local officials. In the months that followed, the girl showed no sign that she intended to head for Syria or Iraq. She took off her veil to go to school, as French law requires, and she spent her days close to her mother.

Then in October, she disappeared. The surveillance cameras in the Mulhouse airport, 50 miles from her home, showed her moving confidently and alone as she used her older sister’s passport to fly to Istanbul and then on to Gaziantep, a Turkish city known as the gateway to Syria for jihadists.

Government officials say they believe that the girl, whose first name is Soukaïna but whose last name cannot be published under French law, became one of the small but growing number of adolescent girls who, seduced by Internet recruiters, have shown amazing determination in their efforts to join Islamic jihadists.