French intelligence agencies knew as far back as 2009 that Ismaël Omar Mostefaï, one of the three suicide bombers who attacked the Batalcan concert hall in Paris, had been radicalised in a group in France led by a veteran jihadist with a history of planning terrorist attacks, Mediapart can reveal. Mostefaï had also been spotted with the group when it was under surveillance in April 2014, and the authorities were later informed that he had almost certainly gone to Syria, at the same time as another future Bataclan bomber. But by late 2014 the secret services no longer knew of his whereabouts. He did not resurface again until November 13th, 2015, when he was part of the coordinated attacks that killed 130 people in Paris. The French authorities, however, deny there was any intelligence blunder. Yann Philippin, Marine Turchi and Fabrice Arfi report.