For months now, Western counterterrorism experts have sounded the alarm: as ISIS loses ground, foreign fighters from America and Europe may try returning home. When they do, the experts cautioned, they will carry the terror threat with them, ready and willing to strike. Law enforcement needs to be prepared.

Now, with the fall of the Iraqi city of Harija, the Islamic State's last major stronghold, and the impending collapse of its Syrian capital, Raqqa, the time has finally come. But is law enforcement prepared?

Not really.

An estimated 5,000 Europeans joined ISIS and other terrorist groups since fighting first broke out in Syria. While some surviving members may choose to remain in the region, or travel to other conflict areas like Afghanistan, a few thousand others are likely to try to make their way back home. In countries such as France, Belgium, Germany and The Netherlands, most of them are at the border, where they will be taken into custody and ultimately tried for terror-related offenses.

But that covers only those who re-enter through legal means. Most will try clandestine paths. And porous borders make such under the radar re-entry disturbingly simple, as smuggling of refugees has already made abundantly clear.

An ongoing lack of coordination of intelligence and security agencies among European countries further enables terrorists to slip in unnoticed. That's what happened with Mehdi Nemmouche, who shot and killed four people at the Brussels Jewish Museum in 2014. Reda Kriket travelled twice to Syria before being arrested in March 2016 on suspicion of plotting "at least one" attack in France. And Brussels-Zaventem Airport bomber Ibraham El Bakraoui had been previously arrested by Turkish officials as he attempted to reach the Islamic State.

"Despite improvements since 9/11, foreign partners are still sharing information about terrorist suspects in a manner which is ad hoc, intermittent, and often incomplete," a 2015 report of the Task Force in Combating Terrorist And Foreign Fighter Travel said.

In a typical example of European intelligence failures, though Dutch authorities learned of that arrest through the FBI weeks before the attacks, that information was never turned over to Belgium.

Such critical intelligence slips do not pose risks only to Europe. "The larger concern is that some European extremists might be able to make it to the United States undetected once they have left the battlefield," state the report's authors.

With visa-free entry into the U.S. for European citizens, such fears are not unfounded, and compound the threats posed by returning U.S. nationals.

Even so, some U.S. officials caution against over-reacting to the threat American returnees may cause. Former Director of National Intelligence James Clapper argues that most of the 40 Americans who have returned from the Islamic State did so for reasons "that don't relate to plotting" – such as homesickness and family matters. Others, like analysts at the New America Foundation put a greater emphasis on the danger from jihadists who never actually left the U.S., but were radicalized locally or online.

But not everyone agrees.

"Whether or not returning fighters carry out attacks, they return with the prestige of warriors and credibility on the street," a 2014 RAND report observed. "They are able to recruit other fighters to go to the Middle East and they can gather like-minded groups around them." RAND political scientist and International Center for Counterterrorism fellow Colin Clarke agrees, writing in the Atlantic that some returnees, both in the U.S. and in Europe, may "attempt to resuscitate dormant networks, recruit new members, or conduct lone-wolf style attacks," as some Europeans have already done.

Tackling the threat, U.S. and European officials agree, depends heavily on European authorities' ability to improve their intelligence capabilities – a daunting task at a time when rising crime rates, concerns about existing terror threats, logistically complex bureaucratic systems, and international rivalries continue to draw on their reserves. Moreover, not all European states agree on the best way to handle returnees. While most are jailed and tried on terror charges, returnees to Denmark, for instance, are placed in rehab programs that offer schooling, job training, and housing, among other benefits – the hope being that they will rejoin society and reform.

Moreover, the EU Radicalisation Awareness Network put forth a series of recommendations in July for all member states. Countries should:

· Create "re-socialization" programs for detainees even pre-trial.

· Provide religious guidance through "trustworthy chaplains."

· Employ mental health experts to work with those who may suffer PTSD or "disillusionment."

· "[B]e aware that many returnees – even if not engaged in criminal behavior – may still strongly support ideologies opposing apostates, other religions, so-called infidels, women's rights and even EU societies as such. Most have been subject to severe indoctrination. Consider dialogue, mentoring and other techniques for returnees with such strong beliefs."

In part, this careful handling reflects the reluctance of countries like Denmark to treat returnees as criminals. But as RAND points out, it also comes as a result of difficulties in assessing "which returnees pose a terrorist threat and which do not." These uncertainties could require monitoring all returnees for indefinite periods, raising civil rights questions as well as economic and other practical concerns.

Whether such careful handling will effectively thwart radicals remains to be seen. Meantime, U.S. officials are taking a different, more punitive approach. Sentences are longer – and can include the death penalty, which is banned in Europe. Moreover, EU laws can be vague regarding which potentially terror-related activities constitute a crime. By contrast, RAND says, American courts are better prepared for the challenges of convicting returnees, thanks in part to the fact that in the United States, "providing material support to a terrorist organization, which includes joining or attempting to join a terrorist group, is already a crime."

But for both Europe and America, RAND suggests another alternative: encouraging returnees to cooperate with intelligence agencies in exchange for lighter sentences, thereby becoming valuable resources in the battle against jihadism in the homeland, helping to locate and convict other terrorists returning from abroad, especially those who have entered illegally. These are the hardest to find and pose the greatest danger.

But even without cooperation from within, there may be other strategies. Rand's Colin Clarke calculates that approximately half of all Islamist terrorists involved in attacks in Europe had criminal pasts. In contrast, others put the figure closer to 22 percent. "Many foreign fighters began as criminals," Clarke writes on Lawfare, "and many might turn to crime on their return." Indeed, those seeking to finance attacks are likely to look to criminal activity – such as drug trafficking – to secure it.

Hence, he notes, "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."

Clarke's approach for Europe applies equally to America. Yet for all its practicality, the criminal element also points to an additional danger: as Canadian counterterrorism analyst Mubin Shaikh has noted, "prisons are a factory for radicalization. The jihadis themselves say prison is the university of jihad." Should those trained in militant jihad find their way back into the criminal circuit, that situation could only worsen.

Ultimately, the problem of returning jihadists promises to be a complicated one across the West, likely for years to come. In the face of an unprecedented terror threat, we have no map to travel by. But in the words of Swiss author Ella Maillart, "the sooner we learn to be jointly responsible, the easier the sailing will be."

Abigail R. Esman, the author, most recently, of Radical State: How Jihad Is Winning Over Democracy in the West (Praeger, 2010), is a freelance writer based in New York and the Netherlands. Follow her at @radicalstates.