Michael Delefortrie, who joined a terrorist group in Belgium and travelled to Syria to become a jihadi fighter, is pictured here after his return to the E.U., leaving a court in Antwerp. PHOTOGRAPH BY FRANCOIS LENOIR / REUTERS

Michael Delefortrie grew up in a secular Christian household in Antwerp, Belgium, but secretly converted to Islam in 2006, when he was seventeen. One day, he came home from the mosque to discover that his father had dug up his Koran and prayer rug, and placed them on the table as props for the heated dispute to come. “I was a little bit angry that he touched the book,” Delefortrie told me, “because I know it’s a sacred book,” and now it was sullied by his father’s touch. His father was angry too. Delefortrie told me he issued a cruel ultimatum: “If you want to be a Muslim, go.” The teen-ager, who had A.D.H.D. and was trying to stop using alcohol and drugs, moved into an apartment above the mosque. He lived there for the next two years.

Elsewhere in Antwerp, a petty criminal named Fouad Belkacem began to gain notoriety for delivering fiery homophobic rants in public squares, and for demanding that Belgium become an Islamic state, governed by Sharia law. He quickly established a following of young men, named the group Sharia4Belgium, and plugged into an international network of jihadis striving to dismantle liberal European values and institutions. (I wrote about one Sharia4Belgium member, Jejoen Bontinck, for the magazine.) Delefortrie became one of Belkacem’s enthusiastic devotees. In December, 2011, he was arrested for trying to sell a Kalashnikov online. After being temporarily shunned by the group for having drawn too much negative attention, he created a splinter organization called Sharia4Flanders, but never managed to secure the interest of a second member. The following summer, the first Sharia4Belgium member left for Syria. Several dozen others had followed before Delefortrie left home.

In December, 2013, Delefortrie boarded a bus from Antwerp to Cologne, Germany, then took a taxi to Dusseldorf. From there, he flew to Istanbul, Turkey, then south to Adana, near the Syrian border. He paid a smuggler, hopped an unguarded patch of wire fence, and, now in Syria, met up with a Belgian friend, who drove him to an ISIS base in Aleppo. After being questioned about his reasons for coming to Syria, Delefortrie was transported to a large, walled villa housing foreign ISIS recruits. He lived there, among Tunisian, French, Belgian, and Dutch fighters, for five weeks, occasionally updating his Facebook account with pictures of himself dressed in camouflage and gripping a Kalashnikov, until moderate Syrian rebels attacked the villa. He and the other ISIS fighters fled. While the Syrian rebels pilfered their money and belongings, his group took refuge for a couple of days in an abandoned Carrefour shopping mall. Then, “We attacked them,” he told me in a dimly lit bar in Antwerp, this winter, before quickly revising his story: “_They”—his comrades—“_attacked them,” he emphasized this time.

Shortly after the battle, Delefortrie came back to Belgium, where, a few weeks later, he found himself in an interrogation room, seated opposite federal police. He told them repeatedly that he had never participated in the armed struggle, insisting that he only left for Syria to seek a “better life” and to provide “ideological support.” He dismissed the online pictures of himself carrying guns as “pictures to brag,” and denied any knowledge of a video posted to his Facebook account, titled “ISIS mujahid gives some advice,” claiming, “I don’t know what this movie is about.” Six other Sharia4Belgium members also returned from Syria, some of them offering even flimsier excuses. One claimed to work for the U.N.H.C.R., the U.N. refugee agency, but, when asked to give the full name of the organization, he told police that the first letter “probably stands for United, but I don’t remember the rest.” Another said he had been an ambulance driver, but could not name a single aid organization operating in northern Syria. A third, who confessed to joining a jihadi group that kidnapped, ransomed, and murdered local civilians, swore he only carried out menial tasks, telling police, “I just assumed if a bomb fell on the house while I was doing the dishes, I was also a martyr.” Mark Eeckhaut, a Belgian crime reporter, joked over beers in Antwerp, this winter, “If you believe the guys who are in this trial, nobody is fighting in Syria. Everybody’s cooking.”

The federal police knew far more about the group’s activities in Syria than Delefortrie and the others understood. Months before the first Sharia4Belgium member took off for Syria, in August, 2012, the police had opened an investigation into the group. At that time, they were concerned about a possible attack on Belgian soil. They intercepted thousands of phone calls, text messages, and e-mails, and continued to gather evidence as members travelled to Syria, seeking martyrdom and blood-fuelled conquest. Several Belgians in Syria made frequent calls to friends and family members back home, in which they described the dynamics, training, and activities of the jihadi group most of them joined, the Mujahideen Shura Council, which was eventually absorbed into ISIS. In January, 2013, police officers listened to Hakim Elouassaki, a Sharia4Belgium member, tell his girlfriend that he had murdered a kidnapped Syrian civilian that morning. The Syrian man’s family had paid Elouassaki a ransom of thirty thousand euros, but he had asked for seventy thousand. “As I shot him, he put up his hand,” he said, “so the bullet went through his hand and his head.” When some Sharia4Belgium members returned home, there was sufficient evidence to arrest them, and, after the first or second uncoöperative interrogation, some began to give exhaustive descriptions of what they had seen. One made such detailed sketches that the Belgian police were able to pinpoint the location of an extravagant palace belonging to Amr al-Absi, whom the U.S. State Department said was “in charge of kidnappings” for ISIS.

Having amassed tens of thousands of pages of evidence, the Belgian government charged Sharia4Belgium members collectively as a terrorist group, with Belkacem as its leader, and prosecuted members individually for various other crimes. Most members were still in Syria, and some were dead, but Delefortrie and the six other returnees diligently attended court last fall. (Only Belkacem had never gone to Syria). It was the largest terrorism trial in Belgian history. On some days, desperate parents, whose children were still in Syria, shouted accusations at Belkacem from the audience. Though European prisons are known hotbeds of radicalization, the authorities saw no room for a soft approach in dealing with Sharia4Belgium members. “Once you know, like we know, that people are part of killing civilians in Syria, it’s not a question,” a Belgian federal-police official told me. “If you have that information, you have to go for the repression.”

In this diptych from Belgian federal-police files, a photograph of Amr al-Absi’s palace in Syria appears on the left, with the sketch of the location provided to the police by a returned jihadi on the right.

In the last few years, more than four thousand Europeans have abandoned their home countries for the jihadi battlefields of Syria and Iraq, and close to a thousand have quietly returned. Many are questioned by police and intelligence services, but the number of prosecutions among the European Union’s twenty-eight member states remains shockingly low. A memo penned by Gilles de Kerchove, the E.U. Counterterrorism Coördinator, says that, as of December, there had been “around ten convictions” of foreign fighters with European citizenship or residency. “The judicial response,” he noted dryly, “does not reflect the scale of the problem.”

In Europe, Denmark is second only to Belgium in jihadi fighters per capita, but the two countries’ approaches to returnees have diverged sharply. The Danish Security and Intelligence Service has estimated that at least one hundred and fifteen Danes have fought in Syria, most of them with ISIS. Some have died, but roughly half are now back home in Copenhagen and Aarhus, Denmark’s second-largest city. It is a crime, in Denmark, as in Belgium, to join or train with any terrorist organization, whether or not the government can prove a violent act was committed in a distant war zone. Still, charges have not been publicly filed against a single Danish returnee.