"If a miracle happens and my son returns home, he will face jail time — and I'm okay with that."

A British woman tells me this over coffee in September. Her son has been fighting with the Islamic State (IS) for months, and he contacts her only sporadically. The UK government has been honest with her about the possibility that he will never return, and the fact that he will face prosecution if he does.

"If he comes home, he will get to stay alive and may at least have a chance at life in the UK, rather than the death sentence ISIS has given him," she says. "Right now, our family just waits for that call telling us he's gone."

The Paris attacks made what had been a largely hypothetical fear in the West very real: that extremists who left their own countries to join militant groups like IS in Iraq and Syria could return as trained fighters and launch attacks. The fact that at least six of the Paris attackers left their homes in France and Belgium, likely fought in Syria with IS, and were then able to freely return to Europe — even though the majority of them were no doubt known by law enforcement and intelligence agencies to be extremists who traveled to the Middle East — should force those agencies to think more deeply about their blind spots.

For more than a year now, I have been studying the foreign fighter phenomenon in Syria and Iraq, attempting to understand individual motivations leading Muslim youth to undertake such a radical commitment. I have spoken to dozens of Western jihadis currently fighting with a variety of militant movements, as well as with their supporters, parents, and close friends.

One thing is clear: There is no typical jihadi.

These youths left their homes for diverse reasons, traveled to different places, and fought with a surprisingly wide array of militant groups. The youths differ in age, gender, psychology, and ethnic and religious backgrounds. The reasons for their eventual return to their home countries will be similarly diverse. With IS in particular, we are not dealing solely with jihadis, but also with young girls, children, and entire families who have left to live under the so-called caliphate. While some of these people should certainly face prosecution, it may not be the right approach for all of them.

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I have identified three distinct groups who return home after leaving to join IS and other militant groups. The first — like those who launched the Paris attacks — are what I call operational returnees. These fighters left their home countries to join jihadi movements in Syria and Iraq, and were then tasked with returning home to launch strikes. Paris attacker Bilal Hadfi, for example, left Belgium for Syria in February 2015. According to a friend of his family, Bilal stopped communicating with his mother by July. The next time she saw his face was on the news; he had just blown himself up outside of a McDonald's near the Stade de France. Operating in small cells in major cities, these returnees may inspire and mix with homegrown radicals to launch an attack, with explicit or tacit help from the group's leadership, whether it be al Qaeda or IS.

Another group we are likely to encounter are the disengaged returnees. These individuals still express admiration and allegiance to global jihadism, but have left the fight for reasons unrelated to the movement. One returnee I interviewed in Belgium, who was given a suspended sentence upon his return, still expressed his commitment to IS, but had returned to normal life at home. Others go to Syria to fight against the regime of Bashar al-Assad, only to realize that much of their energy is spent on other things, like fighting rival jihadi groups. While they may rescind their allegiance to a particular militant group, they remain committed to the broader cause of jihadism (even if launching attacks on the West may not be their primary focus).

For instance, I have spoken to individuals who left Syria to get married, or who left because their mother was ill in their home country, or who were simply exhausted by war. There is also some evidence, based on interviews, of female IS members who express a desire to leave after their husbands are killed in the fighting.

The final group are the disillusioned returnees. They left to join the so-called caliphate looking for utopia and found something very different. They are now returning home to their families — or trying to. They may reach out to their parents to help them get out of the region. Or they contact smugglers and even members of other militant groups operating in Syria for assistance. As one Canadian mother told me last month, "every time I talk to my daughter, I get the feeling that she wants to come home. But she can't be honest with me because her husband is always around."

It is critical that even in the aftermath of the horrific Paris attacks, we do not simply crack down on all returnees. We must remember that many foreign fighters have already returned to their countries of origin, from a variety of different conflicts, and that most are living peacefully. Some of them left their homes as young people, then realized they'd made a mistake. That does not mean they are innocents, nor that they should all be exempt from suspicion, surveillance, or imprisonment. The challenge for law enforcement is to develop a way to accurately gauge the threat posed by individual returnees. Not an easy task by any means — the process of data collection on returnees is still in its infancy, and programs around the world dealing with returnees have thus far shown only mixed results.

I pose this dilemma to the British mother as we finish up our coffee.

"I would say to people who see my son as a threat," she says, "that it is all too easy to demonize these kids as a way of distancing ourselves from the problem. I would say that these kids are victims also."

As such, our approach to returnees, while cautious, should also hold out the possibility of reform and reintegration.