The list of atrocities committed by ISIS continues to grow, with the latest being a chilling pamphlet that details the organization’s policy on treating the women they kidnap and then use as sex slaves. This is the latest account of ISIS’s dealings in kidnapping and human trafficking in which they target women and children, often from the minority Yazidi religion, and sell them for as little as $25 or keep them as slaves.

ISIS is not the only terrorist group to engage in kidnapping and trafficking. Just a few days ago, Boko Haram kidnapped 200 villagers and killed dozens more in Nigeria, further terrorizing the already tormented community. Indeed, human trafficking plays a growing role in the operation of 21st-century terrorist organizations.

Several years ago I gave a public lecture on the topic and mentioned a case that is in the first chapter of my new book, Dirty Entanglements: Corruption, Crime and Terrorism. The White Lace Case in Los Angeles involved women from the former USSR trafficked into high-end prostitution. Many of the women arrived in the United States as part of sports and religious delegations. In order to extend their legal residence in the United States, they had to obtain other visas. One of the leaders of this trafficking ring registered the trafficked women as students at a language skills school, thereby obtaining “student visas” for the prostitutes in her organization. The language school did not focus on providing instruction but instead was a visa mill. This same language school also provided visas to the 9/11 hijackers. In other words, the 9/11 hijackers and the trafficking victims shared the same “facilitator.” This facilitator was a point of intersection of crime and terrorism.

When I finished this talk, a government official approached me. He informed me that he was on a task force studying human trafficking and his role was to find the links between trafficking and terrorism. In his months in this position, he had not found a single example such as this. He asked how I found it. I answered that I had gone and talked to many members of law enforcement who through their investigations understood these links.

At that time, pre -9/11, the links were more subtle and had to be hunted down. But this case, already 15 years ago, shows that there were links at that time between human trafficking and terrorist activity even in the United States. Today they are more direct, especially in many conflict regions of the world. Yet policymakers focus nearly all their attention on more visible crime-terrorism links—primarily drug trafficking—and miss the important links between human trafficking and terrorist organizations.

Human trafficking now serves three main purposes for terrorist groups: generating revenue, providing fighting power, and vanquishing the enemy. For terrorists, human trafficking is a dual-use crime like drug trafficking and kidnapping. It not only generates revenue, but it decimates communities. As we see in Nigeria and Iraq today, trafficking intimidates populations and reduces resistance just as enslavement and rape of women were used as tools of war in the past.

Trafficking and smuggling are part of the business of terrorism, and constitute one activity in the product mix of terrorist groups. Terrorists smuggle drugs, arms, and people. Maoist insurgents in Nepal have exploited the long-standing trade of young girls taken from their country to the brothels of India to finance their activities. Evidence suggests that the LTTE smuggled Sri Lankans to finance their activities and the PKK exploited the porous mountain borders in eastern Turkey to facilitate human smuggling from countries in the Middle East and South Asia. Cells of the Ulster Volunteer Force of Northern Ireland received narcotics as payment from Chinese “snakeheads” in support of their smuggling networks. German authorities in 2006 arrested an Iraqi and a Syrian who smuggled individuals from their home region and were suspected of having links with the Ansar al-Islam terrorist network.

While trafficking and smuggling does generate revenue, they are not central money-making endeavors for terrorists and are committed primarily for other reasons. Pakistani terrorists buy children to serve as suicide bombers. Rebels in Africa trade in children to fund their conflicts and obtain child soldiers. More recently, Boko Haram shocked the world by kidnapping 276 female students and threatened to traffic them. ISIS members have taken young Azidi girls, raped and sold them off for trivial prices. The girls and women may sell for as little as $25 and sometimes even less, suggesting that this is not a revenue-generating operation when a million dollars daily is gained from oil sales. Rather, human trafficking, like slavery in the past, is a way of demoralizing the conquered.

Those not in the direct sight of terrorist groups may also become victims of human trafficking, even as they flee to safety. People displaced by terrorists are vulnerable to trafficking—both sexual and labor. Young girls fleeing with their families from the Syrian conflict today have been trafficked in Jordan and other neighboring states, just as occurred with earlier waves of refugees from Iraq. In Turkey, crime groups in border areas are exploiting the labor of Syrian male refugees who cannot find legitimate employment. Many more illegal migrants face labor trafficking in Europe as they flee the conflict regions of North Africa and the Middle East.

Human trafficking was once a crime associated primarily with a range of small to large crime groups. But as terrorist groups begin to function more as businesses, we unfortunately observe the expansion of terrorist groups into this criminality. Historically, conquering armies have seized inhabitants of conquered areas and enslaved them. But what is different is that traditional practices of the past have been combined with the business acumen of terrorist groups today. In their effort to diversify their revenue, they have capitalized on traditional practices to new advantage. Women and children are disproportionately victims, but they are not alone. Exploitation of trafficking victims may be most acute in conflict and adjoining regions, but it is not confined to these areas.

To combat the link between human trafficking and terrorism, we and European allies must prioritize this problem. Unfortunately, almost all attention goes to the drug trafficking-terrorism link without addressing the role that human trafficking plays as a terrorist funding source and a destroyer of communities. This must be added to our national security priorities and those of NATO. But it also must be given much higher priority by non-governmental organizations and peacekeepers who intervene in conflict regions. Moreover, recipient countries of refugees fleeing terrorist conflicts must prioritize anti-trafficking efforts to ensure that traumatized refugees are not subject to further victimization. Support for refugees must include anti-trafficking measures that include targeting traffickers and support to safeguard women and children.

Louise I. Shelley is a professor at George Mason University and director of the Terrorism, Transnational Crime, and Corruption Center (TraCCC). She is the author of Dirty Entanglements: Corruption, Crime and Terrorism (Cambridge University Press).