U.S. authorities this week disrupted a Minnesota-based ISIS recruitment cell working to send young Americans overseas to fight in Syria, highlighting a grave and growing threat from home grown radicals who are seeking to join extremist groups overseas. But despite these arrests, we are nowhere close to shutting down the jihadi superhighway that has allowed thousands of young people to shuttle between the West and the world’s deadliest terrorist sanctuary.

It’s disturbingly easy for violent fanatics to get to and from the conflict zone undetected, and security gaps in Europe are a big part of the problem. 

The news comes only days after a 23-year-old Ohio man was charged with plotting a terrorist attack in the United States after he returned from Syria. Over the course of just two months last spring, the suspect managed to book a one-way ticket to Greece, slip across the Turkish border into Syria, train with an Al Qaeda-linked group, and return to our country after being dispatched to conduct an attack.  Despite expressing support for ISIS and other extremists on social media, it doesn’t appear he was stopped in either direction. 

It’s disturbingly easy for violent fanatics to get to and from the conflict zone undetected, and security gaps in Europe are a big part of the problem. 

More worrisome, intelligence officials have publicly estimated that at least another 40 Americans have returned to the United States after having joined extremists in Syria, and thousands of Europeans who are still over there fighting have visa-free access to our country. 

It’s disturbingly easy for these violent fanatics to get to and from the conflict zone undetected, and security gaps in Europe are a big part of the problem. 

A recent ebook published by an ISIS supporter advised aspiring combatants to travel through tourist hotspots like Spain and Greece—“or any European country”—because they will be less likely to be detected in route to Syria from those locations. Indeed, thanks to the 1985 Schengen Agreement, most of the border checkpoints between European countries have been eliminated.

For instance, the man known as "Jihadi John," the masked operative responsible for many ISIS beheadings, reportedly travelled freely through Europe despite being on an anti-terror watchlist, much like Hayat Boumeddiene, an associate of the Charlie Hebdo terrorists. The latter was on the radar of French police but managed to avoid detection by simply driving to Spain, where she boarded a flight for Turkey. “I had no difficulty getting here,” she boasted in an ISIS-published interview from Syria.

Extremists are exploiting European security gaps to return from the conflict zone, too. Tens of thousands of illegal aliens from Syria are flowing into Southern Europe by boat each month, and already ISIS claims to have used these routes to sneak operatives into the West. Sadly, authorities in places like Italy are often turning a blind eye, hoping the immigrants will flee into the heart of the continent and become another country’s problem.

To many, Turkey is ground zero because it is the primary backdoor into and out of Syria. Its lax traveller screening policies and porous border have allowed international extremists to move freely over the border, despite the government’s attempts to step up security. But Europe’s vulnerabilities are not confined to Turkey. They are widespread.

Europe has struggled to identify terrorists, keep them from traveling, and detect them when they cross borders.

For instance, many European states have slashed their intelligence budgets in recent years—making it harder to detect radicalized individuals—and intelligence agencies in many states don’t share information with each another, increasing the odds a terrorist will evade authorities.

Even when they do identify extremists, European partners have had difficulty keeping them from leaving to join terror groups in Syria. While some are confiscating the passports of would-be jihadis, revoking their IDs can also drive suspects to purchase fake or stolen documents, and sadly, many European countries only screen a fraction of passports against fraud databases—a blatant vulnerability.

But the Achilles Heel is Europe’s weak terrorist watchlist capability. EU law forbids blanket screening of citizens, meaning that EU nationals are rarely checked against terrorist watchlists when they travel—even though thousands of them have fought alongside extremists in Syria.  Compare this to America, where a U.S. citizen flying home is checked run against the watchlist at multiple stages.

This glaring weakness is allowing potential terrorists to fall through the cracks, and although officials are talking about solutions, Europe’s slow bureaucracy may take months—even years—to fully implement one. How many more extremists will travel undetected through Europe in the meantime? And how many of them will ultimately seek to come to America?

Taken together, these security gaps pose a serious threat to the United States and not just because U.S. extremists are using Europe as a transit hub to join ISIS and Al Qaeda. Most concerning are the EU citizens who have joined violent extremist groups and whose passports can get them into the United States with relative ease. If European authorities cannot identify these fighters to begin with or detect them when they travel, they are more likely to reach our shores unnoticed, too.

Make no mistake: we can and must do more here at home. The Obama administration is far behind the curve in scaling up programs to counter domestic radicalization, and it has struggled to explain to Congress whether it has a strategy is to combat terrorist travel. This is why I recently launched a bipartisan Congressional Task Force on Combating Terrorist and Foreign Fighter Travel to examine our own security gaps.

But we should not be playing defense at our one-yard line. To protect America, we must push our border security outward. That means working with our foreign partners in Europe and beyond to urgently fix their security deficiencies—before more extremists leave the battlefield and set their sights on our city streets.

Republican Michael McCaul, represents Texas' 10th Congressional District in the U.S. House of Representatives. He serves as chairman of the House Committee on Homeland Security.