Editor’s Note: As the Islamic State's territorial control shrinks and its prestige declines, many volunteers will try to find their way home—perhaps to conduct terrorist attacks there on the group's behalf. Colin Clarke of the RAND Corporation argues that one of the most productive ways to identify and disrupt returning fighters is to focus on the criminal underworld. Many foreign fighters began as criminals, and many might turn to crime on their return.

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Now that the Islamic State’s caliphate is collapsing in Iraq and Syria, Europe is bracing for an influx of foreign fighters attempting to return home to their respective countries of origin. Many of the Islamic State’s European cadres have a criminal past, and tracking and arresting them in the criminal underworld will be vital for preventing potential attacks and longer-term counterterrorism success.

To date, European government officials have not identified one clear or common route back to Europe for returning foreign fighters. But inevitably, foreign fighters will return to their home countries—remaining in Iraq or Syria is becoming more untenable with each passing month. To be sure, some returnees will be met at the airport by police or security officials where they might be arrested or subjected to questioning. Others may return unnoticed, but either report themselves to authorities or be reported by family members or acquaintances. By far, the most troubling returnees are those who return surreptitiously with the intention of conducting attacks in their home countries or elsewhere in Europe.

European government officials have not identified one clear or common route back to Europe for returning foreign fighters. But inevitably, foreign fighters will return to their home countries...

Since the foreign-fighter phenomenon began in 2014, significant research has been dedicated to determining how Westerners intent on jihad earned money to travel to Iraq and Syria. Yet there is a complete dearth of analysis attempting to discern how returning foreign fighters might finance their trips home and how they would seek to earn money once they successfully returned. This has become an even more pressing issue in the past six months as the Islamic State’s territory has been recaptured and its fighters killed, arrested, or forced to flee.

But countries can take steps to begin mining existing data and publically available information to attempt to prepare for the return of foreign fighters. Indeed, many European governments know who the majority of their foreign fighters already are, having gleaned this information from social-media accounts, community reporting, and intelligence intercepts. As such, the number of truly unknown foreign fighters in many countries is likely a tiny fraction of the overall number of fighters who have traveled to Syria and Iraq.

The approximate number of total returnees and those still remaining abroad has been well-documented. Research from the International Centre for Counter-Terrorism-The Hague from April 2016 estimated the number of total foreign fighters from major European countries, including the total number, those suspected of remaining abroad, those confirmed dead, and those who have returned. By studying the demographics and backgrounds of those who have fled to the Middle East, it may be possible to construct a more accurate profile (e.g. place of residence prior to departure, occupation, known social networks, criminal record) of a returning foreign fighter. While this profile will be just a representation or sketch, and should not be mistaken for a foolproof algorithm, it could provide law-enforcement authorities and intelligence officials with a logical starting point in ferreting out potential returnees.

The report indicates that hundreds of European fighters remain abroad. But since this research is well over a year old, those numbers have likely decreased, with more jihadists returning home and others being killed in fighting. The more interesting number is the hundreds of foreign fighters who have already returned. Belgium, France, Germany, and the United Kingdom top the list of most returnees, although Austria, Denmark, the Netherlands, and other countries also have significant numbers.

In March 2017, the Council of the European Union adopted new rules that strengthen and widen the scope of existing legislation, criminalizing the following actions: traveling within, outside, or to the European Union for terrorist purposes; the organization and facilitation of such travel; training and being trained for terrorist purposes; and providing or collecting funds with the intention or knowledge that they are to be used to commit terrorist offenses. But there is no monolithic policy across all countries within Europe for dealing with the return of foreign fighters, or for that matter their families. Instead, existing laws range from punitive to rather forgiving, depending on the extent of an individual’s involvement while overseas.

Where European countries may have some flexibility is in dealing with returning citizens who can prove passive involvement in the conflict. Here, it may be possible to offer reduced punishments for returnees willing to provide valuable information to law-enforcement and security officials, including what could be valuable intelligence on how they (and people they know) financed return trips home. With this information, authorities can begin constructing a blueprint of how others may attempt to do the same. A worthwhile, though ambitious, follow-on effort would be cobbling together a comprehensive and reliable set of characteristics or activities that could help authorities identify fighters.

Since the Islamic State declared its caliphate in 2014, Muslim foreign fighters, and in many cases their families, began traveling to Iraq and Syria to join what they believed would be an ideal state governed by sharia law. While recruitment manuals such as “Hijrah to the Islamic State,” published online in 2015, provided tips and advice for reaching the battlefield in Syria and Iraq, no such document exists to help them return home to the West. These return trips are more likely to be ad hoc and random. In some cases, family and friend connections will be critical, while other fighters will likely travel via human smuggling routes controlled by organized crime.

If militants are sneaking back into the country with the intention of plotting a terrorist attack, it is safe to assume they will do everything they can to remain under the radar of intelligence and law enforcement for as long as possible. And while returnees with criminal associations will likely revert to earning money through the black market and illicit economy, police often disrupt terrorists while going after ordinary crimes, which could ensnare returned foreign fighters unknown to authorities.

If militants are sneaking back into the country with the intention of plotting a terrorist attack, it is safe to assume they will do everything they can to remain under the radar of intelligence and law enforcement for as long as possible.

Indeed, the data suggest that many European citizens who traveled abroad to fight in Iraq and Syria already had criminal backgrounds prior to leaving to wage jihad in the Middle East. Recruiting foreign fighters with criminal connections was a deliberate strategy of the jihadists, which even posted an ad on Facebook that declared, “Sometimes people with the worst pasts create the best futures.”

Studies of jihadi terrorists in Europe have found that a significant proportion have a criminal history. Of the terrorists sampled in scholar Edwin Bakker’s analysis of jihadis in Europe, about a quarter had a previous criminal record. This percentage appears to have grown in the decade since Bakker’s study as jihadi groups have begun actively recruiting people with criminal pasts. A study by Marshall Center researcher Sam Mullins reinforced Bakker’s finding, reporting that, of 47 cases of jihadist-inspired violence carried out in the West over a four-year period, half of the attackers had a criminal past. Research by Robin Simcox of the Heritage Foundation suggests that of the nearly 60 individuals linked to more than 30 Islamic State-related plots in the West between July 2014 and August 2015, 22 percent had a past criminal record, mostly related to drugs. Moreover, a report from the International Center for the Study of Radicalisation indicates that up to 40 percent of terrorist plots in Europe are at least partly financed by “petty crime,” while a recent study by the U.N. Office of Counter-Terrorism concluded that “a relatively large number” of foreign terrorist fighters “have a criminal history, including drug dealing and violence.”

Individuals who have gone on to conduct terrorist attacks in Europe have long been associated with the underground economy, including banal crimes such as selling counterfeit goods, from cigarettes to knock-off designer clothing, handbags, and luggage. According to the ICSR report, terrorists with criminal pasts have easier access to weapons, can be highly adept at avoiding detection by law-enforcement and intelligence agencies, and are skilled at planning discreet logistics. Critically, they often have a close familiarity with violence that can equate to a lower psychological threshold for engaging in violence, including terrorism. Furthermore, just as they do in prison, foreign fighters who travel abroad to fight can significantly grow their networks, which can help them facilitate future operations in a range of new locales.

The return of foreign fighters with criminal backgrounds places a renewed emphasis on intelligence and law-enforcement cooperation...

Upon returning home to Europe, many jihadists will likely continue pursuing familiar paths and earning money through criminality. After returning from Iraq and Syria, though, they could put that money toward financing terrorist plots. The return of foreign fighters with criminal backgrounds places a renewed emphasis on intelligence and law-enforcement cooperation to determine if a foreign fighter known to have traveled abroad has potentially reemerged in local criminal circles.

To root out returning foreign fighters, authorities should first look to the underworld from which the fighters originally emerged. Criminals inevitably return to what they know best. This stresses the need for closer cooperation between intelligence officials at national (and even supranational) levels, local police, and beat cops. European Union officials might look to allocate additional funding for programs that can provide enhanced training aimed at integrating the various echelons involved in countering this growing threat. The sheer number of individuals to identify, track, and monitor is already overwhelming for European security services, particularly in Francophone countries, but additional manpower and resources can at least partly mitigate this challenge. Undercover police work and sting operations on high priority targets could add a proactive element.

The rise of the criminal-jihadist in Europe in not an entirely new problem, although it has been exacerbated by the pending collapse of the Islamic State’s territory. As Petter Nesser, Anne Stenersen, and Emilie Oftedal have noted, “the Islamized criminal has been a common character in the history of jihadism in Europe ever since the phenomenon emerged in the 1990s.” To be sure, European security services have substantial experience dealing with the threat, but as evidenced by the onslaught of high-casualty attacks throughout Europe—the most recent in Barcelona, Spain—they need to do more.