A week after the Brussels Jewish Museum massacre, French journalist Nicolas Hénin recognized alleged shooter Mehdi Nemmouche as his jailer in Syria, where he and three other French reporters were held hostage by ISIS between June 2013 and April 2014. The identification of Nemmouche as an active long-term member of ISIS suggests the complexity and menace of the cycle by which disaffected Muslims from the West are sucked into anti-Western and anti-Semitic ideologies that can lead them to the battlefields of the Middle East and then back home. It also raises troubling questions about what French security agencies and other European authorities knew about Nemmouche.“I was in a rest-cure clinic in Germany,” Henin wrote in his newspaper Le Point last week, of the moment when pictures of Nemmouche were first published by the press. “They were not good pictures. With a lot of pixels as if they’d been faxed. But the air of familiarity hypnotized me. I said to myself, ‘Impossible, it can’t be him.’ I spent a long night of insomnia with my computer on my knees. Abou Omar was getting to me one more time like a ghost from Syria.”Abou Omar was Nemmouche’s war name while he trained for two years with ISIS as part of a French section of the jihadist army that has taken over large swaths of Syria and Iraq. He is described by his ex-captives as an exceptionally brutal psychopath who delighted in beating his prisoners. He seems to have been especially enthusiastic in his efforts against Syrian detainees, whom he sometimes tortured all night long (the journalists could hear them screaming until the beginning of the morning prayer). With French prisoners, Nemmouche appeared to be more of two minds, sometimes beating them up, sometime coming into their cell to sing French songs (most notably Charles Trenet’s “Douce France”), to comment on Faites Entrer L’accusé (“show in the accused”), a popular criminal TV show he used to follow from his computer, or for friendly chit-chat about his future plans, one of which apparently involved a planned attack on July 14 in Paris targeting French President Francois Hollande (“That day, I will do five times Merah,” he is reported to have said, in reference to Mohamed Merah, the Toulouse killer). At some point, Nemmouche may also have been the jailer of American reporter James Foley, who was detained along with the French journalists and whose gruesome videotaped beheading by ISIS last month was cited by President Barack Obama in his announcement this week of a long-term American military effort targeting the group.Now in jail in Belgium, Nemmouche is defended by two Belgian lawyers, Henri Naquay, who supported the French extreme-right-wing leader Jean-Marie Le Pen in 2007, and Sébastien Courtois, lawyer of the Islamic Belgium Center, who was sued for anti-Semitism in 2009. Both are also lawyers and personal friends with the French comedian Dieudonné.The information published by Le Monde on Sept. 6, which was based on documents passed on this summer by the General Direction of Domestic Security (DGSI) to the antiterrorist section of the public prosecutor’s department, raises a number of disturbing questions:1. When did French and European authorities first learn of Nemmouche’s return to Europe, and what did they know about him prior to the Jewish Museum attack?Nemmouche left Syria in December 2013. European intelligence traces him to Istanbul, from where he left, in February of 2014, for southeast Asia, starting a trip that would take him to Malaysia and Singapore. He then landed in Frankfurt, where German police spotted him and passed his name on to French intelligence agencies. Whether Nemmouche went to France from there—in which case French intelligence missed him—or went straight to Brussels is, at this point, unclear.2. Why is the news of Nemmouche’s involvement in the ISIS kidnapping of French journalists only breaking now?It appears that the four French journalists, who were released by ISIS, made a deal with the French intelligence service not to divulge this piece of information—the official motive being to try to protect other hostages still detained by ISIS, a concern that would have become obsolete after the murders of James Foley and Steven Sotloff. This version of events does not entirely jibe with the fact that the news was first released by Le Monde, who got wind of it through an off-the-record judiciary source. Only after Le Monde broke the news did Nicolas Hénin testify. Another hostage, Didier François, confirmed the news but complained that to public that it was “irresponsible” and “endangered the life of other prisoners.”3. How safe is Europe from returning jihadists with European passports?French authorities appear to have been embarrassed by the news—and not just by the fact that a French citizen could torture other Frenchmen for months. Whether or not Nemmouche came back to France from Frankfurt or went straight to Brussels is almost beside the point: Once back in EU territory, he could move pretty much as he wished, due to the absence of internal borders. One possible measure of the significance of this future threat is how hard French authorities seem to be working to downplay the news, and the Minister of the Interior has denied the existence of a planned terror attack for July 14.***Like this article? Sign up for our Daily Digest to get Tablet Magazine’s new content in your inbox each morning.Marc Weitzmann is the author of 10 books and a regular contributor to Le Monde. He is the former editor in chief of Les InRockuptibles.