Salma Abdel was in many ways a typical teenager. She liked her French fries, spent a lot of time on her iPhone 5, and, according to her brother, made “the best pizzas.” Salma’s father refers to the 18-year-old as his “chérie,” or darling. As the youngest of 10 children in a Moroccan Muslim family who had always stayed close to their home in suburban Paris, she was the last person any of them would expect to do anything radical.

All of which is why that day in April was so painful.

Salma woke up that morning, as she had for the past three years, to head off to a local French public school, where she was pursuing a technical degree in pastry-making. She arrived punctually at 8 a.m. Then, inexplicably, she vanished.

Members of the family (whose names have been changed along with Salma’s) got a call from the driver who normally picked her up after school. She was nowhere to be found. Her brother Ibrahim immediately envisioned the worst. “Some man has taken and raped her,” he thought. Salma was born prematurely at 5 months and had overcome mental handicaps her entire life. Though she had carved out a largely normal existence, he still worried that she was vulnerable to manipulation by men.

As it turned out, his suspicion was partly right. Security cameras showed her walking out the back of the school five minutes after she arrived. She went to the airport, bought a ticket to Turkey, and then, with the help of an online jihadi recruitment network, crossed into Syria to join the civil war. It was a Frenchman on the Internet, already in Syria, who her brother says lured her abroad with heart-wrenching photos of children in the conflict.

Salma is one of hundreds of young people fleeing homes across Europe to join rebel groups fighting in the Middle East, creating a major new security threat for the West and emotional anguish in living rooms from London to Berlin.

Driven by everything from anger at the mass killing of Muslims by the Syrian regime of Bashar al-Assad to a sense of idealism about joining a possible new religious state, young people are leaving the security of their families, their educations, and often their suburban lifestyles to support jihadi groups. Some of them are joining rebel movements linked to Al Qaeda. Others are aligning with even more-radical Islamic cells.

The young people are departing in large enough numbers to stir concern in top policy circles across Europe. According to figures from the International Center for the Study of Radicalization at King’s College London, as many as 2,000 Europeans have already fled to Syria. The French government estimates 700 young people have left from France alone – the largest number from any country on the Continent.

Up to 500 youths from Britain have joined Syrian rebel groups, more than 300 from Germany, and at least 100 from the Netherlands. Many Belgian teens have fled, too, as well as dozens from the United States. An American from Florida was linked to a suicide bombing in Syria in May.

Some of the young people are motivated by a deep frustration that the West failed to respond to the strife in Syria, which has claimed more than 160,000 lives in the past three years. Social media have made them easy prey for recruiters fighting for various factions, including the Islamic State (previously known as the Islamic State of Iraq and the Levant), which is seeking to create its own caliphate. The recruiters promise a life of grandeur.

The exodus includes both young European men and women, those who are Muslim by birth and those who are recent converts to Islam. Some have military training. Others, like Salma, enter into a world in which they are totally unprepared to cope. Many won’t return. For those who do, European governments worry about the threat they pose at home.

The European Union’s counterterrorism coordinator, Gilles de Kerchove, says the deadly shooting of four people at a Jewish museum in Brussels in May at the hands of a suspected French jihadi who’d been in Syria underscores the danger. “[Governments] rank this foreign fighter phenomenon as their No. 1 concern for internal security,” he says.

• • •

Syria is hardly the first country to draw foreign fighters to a war. Thousands of leftists signed up as volunteer fighters against fascism in the Spanish civil war of the 1930s. Fifty years later Muslims joined the fight against the Soviets in Afghanistan. More recently, radical Islamists took up arms in Somalia, in Afghanistan after 9/11, and then in Iraq.

But the accessibility of Syria, combined with the powerful “cyber jihad” it has spawned on social media, are allowing rebel groups to reach out to foreigners in ways and numbers that, experts say, is unprecedented.

It is now so easy to get to southern Turkey from European capitals on low-cost carriers such as Easy Jet that those in the field have dubbed them “Jihadi Jets.”

Once there, the foreign fighters usually don’t need visas. They can easily blend in with the tourists. They take public transport or cabs to the country’s border with Syria. Once across, they head to “European houses,” where they are recruited to fulfill whatever task is needed.

It is a scenario that Daniel Koehler, a family counselor with Germany’s Hayat program, which seeks to prevent radicalized German youths from journeying to Syria, likens to Mexican migrants in the US. They walk across the border one day and the next they are standing on American street corners, looking for day jobs.

Even more unusual about the Syrian war – and what makes it important for future conflicts, including the Sunni-led uprising in Iraq – is how jihadism has become a media phenomenon. Videos distributed by radical groups, with their own YouTube accounts, flourish on the Internet. Preachers appeal to young Westerners with messages of martyrdom and loyalty packaged in rock video formats. It’s a new genre dubbed “popjihadism,” says Mr. Koehler. Recruiters convince young people that they can join something bigger and forget their “little problems” back home, such as unemployment or family turmoil.

Through Twitter and Facebook, Instagram and Tumblr, jihadis easily spread their messages to Westerners. Yilmaz, a broad-smiling Dutch jihadist of Turkish descent, has become a social media sensation. His Tumblr account, called Chechclear

.Tumblr.com, glorifies his stint in Syria as the ultimate adventure. A former Dutch soldier, he posts photos of warfare alongside those of kittens and Syrian children. In one photo, he shows a big bowl of M&M’s, under which he writes: “For those at home nothing but colorful candy. For those in Jihad gold :-))).”

It’s a long way from the secret forums, and often austere messages, that characterized communication from the front lines of earlier conflicts. Pieter Van Ostaeyen, a business analyst who moonlights as a Belgian blogger on jihadism, says he’s come into contact with dozens of Western jihadis from Britain, Holland, France, Sweden, Denmark, Finland, and other countries. He says he has regular contact with Yilmaz from his home outside Brussels. If he wanted to go to Syria, he says he easily could, just as young Salma did, even though she couldn’t get herself to and from school without a driver because of her special needs.

In her case, the Web acted as both a map and a motivating tool – far more than her family realized at the time. Normally, Salma would come straight home after school. Sometimes she socialized with neighbors. Sometimes she baked cakes and crepes.

Most often, though, she would spend hours up in her room online on the cellphone her father, a tailor, had bought her. No one thought anything of it. Until now.

“The problem here was Facebook,” Ibrahim says. Reviewing her account later, he found that she had communicated with other French, Swiss, and Belgian men and women. Being from a moderate Muslim background, a family that her brother assures has not a streak of radicalism in it, she was seeking to learn more about her religion, what jihadism is, and how to live life as a good Muslim.

“We couldn’t suspect her of anything,” Ibrahim says, let alone that it was “terrorists” whom she was “friending.” “We thought she was talking with her girlfriends and boyfriends in school.”

Her profile as a recruit isn’t unusual. Koehler says in the past few months his group has seen a steady rise in the number of women going to Syria. He notes that they are often attracted by a form of “jihadi emancipation” – the ability to travel abroad and marry whomever they please without the consent of their families. They are actively recruited because they play a fundamental role in “resurrecting the caliphate.” “With women – who marry, cook, and raise families – [the jihadists] can establish roots,” Koehler says. “There is more chance that people will stay on, not move to the next battlefield.”

Yet in the Digital Age, more than just curious young Muslim women can be recruited. Jihadis can now reach people from every demographic and social strata – from marginalized second- and third-generation immigrants to middle-class Europeans. It’s what David Thomson, author of the new book “The French Jihadists,” calls the “democratization of cyber jihad.”

But the medium wouldn’t be anything without a message. While religious fundamentalism remains a predominant pull, it isn’t the only one. Mathieu Guidère, a French expert on Islamic radicalism, says the attraction of the jihadi movement today resembles the fascination with communist ideology in the 1960s and ’70s.

“A lot of these young people are looking for a cause, something to engage in,” he says. “They perceive that today the only ideology that is challenging the dominant system ... is Islamism and especially jihadism.... If you want to be a revolutionary person ... you can’t do it in the US. You can’t do that in France, or in Great Britain, or Germany.”

• • •

For every family that has had to deal with the emotional tumult of a young person fleeing to Syria, like the Abdels, there are many others struggling with something more ambiguous: a teen they think is becoming radicalized – but they aren’t sure.

Late last year, Helene Pellier, who is 16 years old, told her mother, Anouk Durand, that she had converted to Islam. Raised in a family of nonpracticing Roman Catholics, Helene, like her mother, had long been fascinated by other countries and cultures, and Ms. Durand, a schoolteacher who lives in the suburbs of Paris, wasn’t initially worried about her daughter’s religious curiosity. But then she saw who was behind it: two schoolmates of Egyptian descent. One was a boy who had stopped attending school so he could pray. “I had a bad feeling,” she says. “I told her to be careful because today there are people who are extremists.”

As the new year arrived, Helene (not her real name) stopped wearing makeup and dressed only in full-length skirts. Later she stopped playing the piano and listening to music as some interpretations of Islam dictate. She began wearing a head scarf. By April she refused to greet men with the French tradition of kissing cheeks or to even shake their hands. The tipping point came in May: She was supposed to begin a four-week internship at a salon to finish her degree in hairdressing, but, after two years with solid grades, she quit the program. “Mom, it’s finished,” she said.

Durand says she rushed to stem the downward spiral. She printed out feminist writings about women in Islam and read them with her daughter. Her daughter insisted she was not radicalized, simply living a Muslim life, so her mother wrote her a two-page letter listing the extreme behavior she observed. She called a psychologist and the police. They told her they couldn’t do anything: conversion to Islam, even a hard-line brand, is no crime. “I felt all alone,” she says.

And then Durand met the parents and relatives of other youths who had converted to extreme forms of Islam. They gathered for a support group started by Dounia Bouzar, an anthropologist who wrote a book on preventing radicalization.

Durand identified with the parents, many of whom were middle-class – doctors and teachers. Ms. Bouzar says 70 percent of those who have contacted her center come from atheist, not practicing Muslim, households, defying the popular notion that mosques are the main source of new recruits. Those at the center urged Durand to call a new hot line France had set up to help families who thought their sons or daughters might head to Syria.

“I didn’t suspect she was going to leave, but the other parents said, ‘You must call. We never suspected either that our children would go.’ Many of them are now in Syria,” she says.

She did call. Authorities urged her to sign an order forbidding her daughter from leaving the country. The family also confiscated Helene’s cellphone and computer, discovering strings of messages from the Egyptian classmate who they learned was her boyfriend and who had left for Egypt in early May. In texts, he had been ordering her to change her behavior – and admonishing her not to listen to her mother because she is a “mécréant” (heretic).

Durand gave state investigators permission to monitor Helene’s calls on a new cellphone. She still talks to her boyfriend daily. Helene (who declined to be interviewed for this piece) has told her mother she has no plans to leave France now. But Durand worries that Helene could join him when she’s 18, in a little over a year. “She is an adolescent, she is in love, and she is brainwashed,” she says.

• • •

As despairing as the experience has been for Durand, her case also poses a challenge for governments across Europe. They are increasingly concerned about young people gaining battlefield experience in the Middle East and then returning home to act as sleepers or outright terrorists. While only a fraction may do so, even a small pool of indoctrinated youths woven into violent Sunni cells stirs fear in state cubicles and cabinet offices.

This winter Manuel Valls, the French prime minister who was then interior minister, called it the “greatest danger that we must face in the coming years.” France has the region’s largest Muslim and Jewish communities and believes it has the largest contingent of foreign fighters in Syria. But the problem isn’t confined to a single country, since much of the European Union today is one travel-free bloc. The prime suspect in the Brussels shooting, Mehdi Nemmouche, is French but returned to Europe through Germany and attacked in Belgium.

“We had feared that those returning from the Syrian conflict might plan attacks here,” said German Interior Minister Thomas de Maiziere in June. “We now know those fears were well founded.”

Governments from Berlin to Paris to Madrid have already arrested suspects working as part of recruiting networks in Europe and have reported foiling planned attacks. Now they are trying to coordinate their efforts Europe-wide, by forming a central database to track citizens who have been to Syria. They have also called on Internet companies to clamp down on jihadi propaganda.

Even more urgent, officials are trying to stop young people from becoming recruits in the first place. The European Commission officially launched the Radicalization Awareness Network in 2011, which links community organizations across Europe that counter violent extremism. Like France, countries are also setting up hot lines for people to call when they detect unusual behavior in family members. The French government is planning to propose legislation that would allow intelligence officials to block suspected proponents of Islamic terrorism from leaving the country.

Sadly, all this might be too late for the Abdel family. It didn’t take long for them to discover where Salma had gone. They checked her phone logs and found a number in Turkey. They called immediately. A man picked up and shocked the family by saying she was en route to Istanbul and would soon be transported to Syria.

In the first weeks they texted with her daily. Now correspondence has dropped off to once or twice a week. Salma assures them she is fine. The Frenchman who lured her there asked her father’s permission to marry her. He refused.

Since then, they have found out she did marry the man. The family is crushed. They fear the jihadis just want her to bear a child for the next war. They feel their little “chérie,” who has overcome so much in life, may be lost.

Ibrahim worries, too, that his family might be in danger. The jihadis know their names. They know where they live.

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He has asked French authorities to help him track his sister in Syria. He even wrote to the FBI, saying he believes only the Americans have the power to stop the situation.

“Everybody helps the Nigerian [girls],” Ibrahim says. “But for our European girls and boys, nobody moves. I call to everybody in the world, help us, help us, help us. Please.”