Ingrid De Mulder Brian De Mulder, then 19, at Ingrid De Mulder's home in Luxembourg, the southern-most province in Belgium. He spent four days there during Ramadan before fleeing back to Antwerp. About six months later, he would cross into Syria.

Like many boys in Antwerp, Brian De Mulder dreamed of being a professional footballer. His favorite team was continental powerhouse FC Barcelona and he sought to emulate its Argentine superstar Leo Messi, proudly wearing “10” on the back of his jersey. He had started playing the game at the age of six and grew into a talented attacker. He planned to make a life of it.

Two years ago, Brian, then 17, was among several youth cut from his local cash-strapped club. He was devastated to be sidelined and distraught at what seemed like the end of a dream, his family told TIME. They couldn’t have predicted what happened next.

Brian was raised Catholic and rarely got into trouble. He was tall and slim, with dark shaggy hair. He was proud of his mixed heritage—his mother is Brazilian—and often wore a white cross around his neck, a childhood gift from her. Shortly after being booted from the roster, a group of Moroccan teenagers invited him to play indoors. He jumped at the chance to the fill the void.

At some point over the next two months, he started hanging around their mosque. They named him Ibrahim, says his older sister, Bruna Rodriguez. Later, he converted to Islam and changed his name to Abu Qasem Brazili. His parents were stunned by the sudden transformation, but the liberal De Mulders tolerated it. “She thought it was a phase of a teenager—of puberty,” Rodriguez, 25, said of their mother, who declined requests to speak with TIME. “‘In six months it will be over,’ she hoped. But it became worse with his age.”

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He was a good student, but tensions flared at school when he told other Muslims they weren’t pious enough; he left three months before earning his diploma. By his 18th birthday, he had taken up basic Arabic and would drop to his knees five times each day to pray. “It was more philosophical back then,” his aunt, Ingrid De Mulder, says. “He was asking about things, wondering about God. It was more like learning. He was not radical at all.”

Around that time, in mid-2011, he had begun snubbing non-Muslim friends—even deleting them on Facebook—and favoring those who were more devout. One of them, a neighbor in the same building named Mohammed, introduced Brian to street sermons led by Fouad Belkacem, ringleader of the local Salafist organization called Sharia4Belgium. The group voluntarily dissolved last fall but maintains a vibrant underground network.

Belkacem, known as Abu Imran, is an Islamist of Moroccan descent, known for his radical politics. Antwerp, Belgium’s second-largest city, is celebrated for its diamonds and museums, its art and its food. But as the civil war in Syria has spiraled over 26 months, with reportedly more than 94,000 people killed and 1.5 million refugees in neighboring countries, Sunni extremist networks like Belkacem’s are thought to be radicalizing youth and pipelining them into the increasingly sectarian conflict to fight against the forces of Syrian President Bashar Assad. The Syrian regime is nominally secular and has long defended the interests of Syria’s minority Alawite sect, to which Assad belongs.

Ingrid is adamant that Belkacem indoctrinated Brian, turning him from a normal teenager with “a golden heart” to “a programmed robot” in just months. The family was so convinced he was headed for jihad that last June, Brian’s mother moved him and his younger sister to Limburg, a remote community in the northeast. It didn’t work.

August was one of the last times Ingrid would see the nephew she watched grow up. It was Ramadan, so he spent days inside, sleeping and praying. “He was explaining what he was believing, but he was still Brian like we know him: Smiling, helpful, generous,” she says. He thumbed the Koran, but mainly passages that Belkacem suggested. When he ate after sundown, she prepared fish because her town didn’t have Halal meat. Over the next six months, Belkacem’s followers would occasionally spring him from Limburg. He would swap brand name clothes for a traditional white thawb and criticize his mother and sisters for dressing too casually. When they wouldn’t convert, he said he’d “drop them like bricks.”

One night in mid-January, Brian fled for good. “I love you, but you will never see me again,” he said to his younger sister. He left his keys and mobile phone behind. The family filed a missing person’s report, but he wasn’t a minor. (Ingrid says they were notified later that Brian was a passenger on a Jan. 23 flight from Düsseldorf to Istanbul.) In late February, Rodriguez was deep down the YouTube rabbit hole, looking for any trace of Flemish fighters in footage seeping out of Syria. She came across a video that appeared to show a cadre of rebels laying down their rifles in an open field to pray. She recognized her brother.

It’s a story that differs little from the dozens of others like Brian known to have taken up arms in the war. The video was proof that he was alive, but also a startling reminder that he could meet death at any moment. Up to 6,000 foreigners are estimated to have fought in Syria since March 2011, experts say, with the majority being Sunni Arabs from Egypt, Libya, Tunisia and Saudi Arabia. Of that figure, no more than 10 percent have hailed from the West. A recent report by King’s College London supports their claim, finding that about 600 Europeans—mostly Muslims or radicalized converts from Britain, the Netherlands, France and Belgium—have made the trip. Aaron Zelin, a fellow at the Washington Institute for Near East Policy, says this is the quickest-ever mobilization of foreigners (especially Westerners) into a conflict. If the carnage continues, Syria’s foreign legion will rival that which fought in Iraq, a force that peaked at around 10,000 foreigners over six years.

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The Internet has sped the recruitment process, bringing in youth who were initially introduced to the idea by family or others in their community. “At night, they’re already online, checking things out, seeing what’s going on, and it’s a very powerful combination,” says Clint Watts, a homeland security fellow at George Washington University. That’s what the De Mulders say happened with Brian. He and others, like 18-year-old Jejoen Bontinck, also from Antwerp, were drawn into extremism through religion. Jejoen’s father recently traveled to Aleppo to find his son, but had no luck. He appears to be fighting alongside Brian with the same aim of toppling Assad and bringing awareness to the plight of civilians under siege. Critics say they’ve fallen under the spell of Jabhat al-Nusra, the jihadist outfit that has been listed as a terrorist organization by the U.S. State Department.

The duo fit the classic type of fundamentalist convert: Vulnerable and impressionable teenagers or college-aged men who became disaffected, feel marginalized, experience discrimination or have difficulties assimilating into society. Seeking some all-consuming cause, they get seduced by what David Malet, a lecturer at the University of Melbourne and author of Foreign Fighters: Transnational Identity in Civil Conflicts, describes as “an existential crisis that has to be won or an entire community is going to be wiped out.”

Right now, that cause can be found in Syria. “The longer the battle goes on, the more the zeal can build up within the foreign fighter recruitment pipeline that motivates these kids to go,” Watts says. Little is known about Brian’s travails in Syria, but Westerners like him are generally considered spies upon their arrival; those deemed trustworthy may not even be listened to or given a weapon.

In early April, the De Mulders held a press conference with Vlaams Belang, a far-right political party, to spotlight the threat of Muslim radicalization in Belgium. Two weeks later, police raided homes in Antwerp, Brussels and nearby Vilvoorde. Belkacem and five other men, who were believed to play a role in pushing teenagers into Syria, were detained. For Brian’s family, that brought little comfort. Those around him have stopped watching the evening news so they won’t see the report if (or when) he’s killed. They want him back, but others less so. European anti-terror officials have warned that fighters who wish to return pose a national security threat and that their welcome isn’t guaranteed.

The De Mulders first heard from Brian in late April. He wrote his older sister a private message on Facebook, a response to several of hers asking him to come home. Unrepentant, he claimed they weren’t his family anymore unless they converted and joined him. Later, he wrote to her again about the atrocities he had witnessed and the lack of available aid for civilians. She replied that he was “famous” as a Syriëstrijder (“Syria warrior”) and that if he really wanted to make a difference, he would do it from Belgium by talking to ministers. He hasn’t responded. (A private message sent to Brian’s account was not returned.)

On May 18, four months after Brian slipped away, the family gathered at Ingrid’s home in southern Belgium to celebrate Brian’s 20th birthday in his absence. Rodriguez says they tried to keep spirits high, laughing and eating some of Brian’s choice fare: French fries, steak and chocolate cake. They await messages from him, as well as more help to prevent others from falling into the same trap, but aren’t optimistic. “Each day, more and more guys like Brian—they are going to Syria,” she says. “It has to stop.”

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