A new report commissioned by the Soufan Center has outlined the threat posed by potential Western Islamic State (ISIS) fighters returning to their countries of origin as the caliphate continues to collapse.

The report, titled “Beyond the Caliphate, Foreign Fighters and the Threat of Returnees,” finds that at least 5,600 men, women, and children from 33 countries who traveled to support the Islamic State have now returned to their country of origin, many of which are in the West.

It also estimates that around 30 percent of the approximately 5,000 people who have gone to Syria from countries in the European Union have returned home. Seven of the 129 who went to join ISIS from the United States are now back on home turf, the report adds.

“It’s a hell of a problem,” Richard Barrett, a senior adviser at the Soufan Center told NBC. “It’s very resource-intensive to identify who these people are, track them down and then to do something about them.”

“We have to be cautious of thinking that all [5,600] would go back and be a problem, but I think it’s fair to assume that probably some will be,” he continued.

The report also outlines how, upon their return, Islamic State terrorists continue to pose a serious threat to national security. It states:

If on return they begin again to feel as rootless and lacking in purpose as they did before they left, then they are unlikely to settle back easily into a “normal” life, and as ISIS increases its external campaign, both through action and propaganda, returnees may be particularly vulnerable to contact from people who were part of the network that recruited them, or appeals for help from ex-comrades in arms.

It also details the potential threat posed by women and children, whose commitment to the caliphate and its ideology is difficult to assess:

A subset of returnees that is even harder to quantify, assess and address is the women and children who joined IS but later went home or were captured and await deportation. Without further research, it will be hard to judge the degree of their commitment to IS and their interest in becoming active rather than passive supporters.

“It’s one of the hardest things of all to rehabilitate children who have been recruited by violent extremist groups,” Barrett said. “Once trained, they are incredibly committed and loyal.”

“They will be a lot of mental health issues,” he continued. “And you can’t just put them in a child detention center—you have to treat them. Some will be turned around quite easily and will follow the next ‘mother figure’ that comes along, but others will be unpredictable, to say the least.”

The report’s release coincides with the caliphate’s continued collapse at the hands of the U.S. led coalition. Last week, coalition forces celebrated the group’s expulsion from the Syrian city of Raqqa, while forces in the Philippines also expelled the terror group from their stronghold of Marawi. Islamic State “caliph” Abu Bakr al-Baghdadi had declared Raqqa the capital of the terrorist state in 2014.

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