Editor’s Note: As the Caliphate collapses, many of its foreign volunteers are fleeing Iraq and Syria. A lot of ink has been spilled (some by me, in fact) on the problem of foreign fighters returning home. However, some of these fighters end up in a third country—not in the Caliphate, but not home either—that is not prepared for the problem. Kim Cragin of the National Defense University examines this huge hole in our thinking. She finds a large problem that demands international action.

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Disturbing reports came out of Malaysia this past August. The country’s security services were searching frantically for sixteen Islamic State terrorists who recently had been deported from Turkey. Turkish authorities had detained these individuals travelling to-and-from Syria, interrogated them, and put them on a plane to Malaysia—but then Malaysian authorities lost them. Perhaps even more disturbing, none of these sixteen individuals were from Malaysia. Turkey sent the detainees to Malaysia because it had visa-free agreements with their home countries. With all of the attention on foreign fighter returnees—individuals who travel abroad to fight in Syria and then return home—policymakers would do well to address the issue of unaccounted for Islamic State deportees who do not go home.

According to the U.S. government, 40,000 foreign fighters from 120 countries have travelled to Syria and Iraq since June 2014. It is difficult to know the exact status of all 40,000, especially after the recent operations in Mosul and Raqqa. That said, 79 of the 120 countries have begun to release information on the whereabouts of their citizens who travelled to Syria and Iraq to fight. Based on these data, 6,957 foreign fighters have been killed on the battlefield and at least 14,910 have already departed the Levant.

It would be easy to assume that most, if not all, of these 14,910 individuals are in prison. After all, beginning in September 2014, a series of UN Security Council Resolutions called on countries to make it illegal for individuals to travel abroad to fight for the Islamic State (UN Security Council Resolutions 2170, 2178, and 2253). But, based on official estimates, only 5,395 (36 percent) of the 14,910 foreign fighters who have departed Syria and Iraq are actually in prison. The rest have either returned home without entering into the criminal justice system or were deported to a third country and have disappeared.

The overarching numbers are somewhat grim. Of the 14,910 foreign fighters who have departed Syria and Iraq, 6,837 (46 percent) have returned home without entering into the criminal justice system. 2,678 individuals (18 percent) are unaccounted for deportees. To put these numbers in perspective, approximately 5,000 foreign fighters travelled to Iraq between 2003 and 2008 to fight against U.S. and allied forces. Almost three times that number have already departed the current conflict, and almost twice that number are back home and security services either have decided that they pose less of a threat or they are otherwise unaccounted for.

Some of the deportees have already conducted terrorist attacks. Most famously, one of the three individuals responsible for the March 2016 attacks in Brussels targeting the international airport and a subway station had previously been deported from Turkey. The attackers killed 31 and wounded over 300. In fact, as of October 31, 2017, the Islamic State had conducted 510 attacks outside of Syria and Iraq; foreign fighters had participated in 138 of these attacks. But not all of these operatives were so-called returnees. Returnees have been responsible for only 51 of the 138 attacks (37 percent) by foreign fighters worldwide. The remaining 87 attacks were conducted by foreign fighters who left Syria and Iraq but ended up somewhere else, not “back home.”

It seems obvious that captured foreign fighters should be returned to their home countries and, once home, should be incarcerated. But even if the legal framework is in place, the policy challenges surrounding foreign fighter deportees are complex. Turkey has carried the burden for identifying, detaining, arresting and deporting most of the foreign fighters who have travelled to-and-from Syria and Iraq. In July 2017, Turkey reported that it had deported a total of 4,957 foreign fighters, specifically individuals who had been caught crossing its border with Syria. For most of these individuals, Turkey was able to arrange for their home countries to repatriate them. But not all. I travelled to Turkey in August 2017 to interview officials on the status and the whereabouts of these deportees. I was told that, under Turkish law, it can detain foreign fighters for up to 12 months. After 12 months, if their home countries will not repatriate the prisoners, Turkey must release them.

This makes sense. Turkey has no interest in becoming the new Guantanamo Bay. It, therefore, seems logical that Turkey would deport any captured foreign fighters that were not repatriated. I was more puzzled about why some countries would refuse to repatriate captured foreign fighters.

Since August, I have examined the issue of what I refer to as unaccounted-for deportees. These are individuals who have been deported to a third country for their ties to the Islamic State, but not been incarcerated. Some of these individuals are foreign fighters and others are not. But all of them have been deported for having ties to the Islamic State. I have discovered that the sheer volume of foreign fighter returnees, combined with local Islamic State fighters, has over-stretched the judicial systems of some countries. This challenge is acute for countries in the Middle East and North Africa, like Tunisia, with high numbers of foreign fighters. 2,929 Tunisians have travelled to Syria and Iraq to fight and 800 have returned home. Only a small portion (35 percent) of these 800 individuals are in prison or under house arrest. But Tunisia also has to worry about terrorism closer to home; it has incarcerated an additional 2,200 local Islamic State terrorists.

These countries simply do not have any more room in their prisons. They do not have the resources to implement de-radicalization programs for prisoners. And, in some instances, they do not have sufficient evidence to prosecute Islamic State fighters deported from Turkey or other countries. As a result, some countries are doing whatever they can to reduce the numbers of Islamic State fighters—returnees, deportees, local recruits—in their territory.

It’s like a scary game of hot potato.

This challenge affects the West, too. British officials, for example, have said that approximately 400 of its 800 foreign fighters have returned home. Of these, 108 were deported from Turkey. 24 percent have been prosecuted. But British authorities also have revoked the passports of 150 individuals with dual citizenship, including foreign fighters and their spouses. Italy deported 135 suspected terrorists between January and July 2017. Germany recently deported two individuals, who had been born in Germany to Algerian and Nigerian nationals, accused of supporting the Islamic State. France has admitted that it hopes most of its foreign fighters die on the battlefields of Syria and Iraq. It’s like a scary game of hot potato.

We need a different approach. The most logical starting point is for the UN Security Council to pass another resolution that addresses the status of foreign fighters captured on battlefields or by third countries. This resolution should outline the responsibilities of security forces that capture foreign fighters.

Specifically, once countries detain or arrest a foreign fighter, all relevant law enforcement sensitive information should be transferred to the fighters’ home countries in a timely manner. Ideally, this information would be transferred bilaterally, but Interpol also could serve as an intermediary if necessary. Standards also should be established for biometrics—e.g. photographs, fingerprints, or DNA evidence—to aid countries as they attempt to prosecute these captured fighters. And, similarly, a process should be established to mediate disputes related to false passports, dual citizenship, or stateless persons.

Beyond foreign fighters, the UN Security Council resolution should call for the creation of a clearinghouse at Interpol for law enforcement sensitive information on all deported terrorists, including those sent home or to a third country. Equally important, the new UN Security Council resolution should strongly encourage countries to repatriate their foreign fighters and discourage the revocation of citizenship.