(CNN) Tuesday's bombings in Brussels left at least 35 people dead and 300 wounded. They are the latest additions to an increasingly long list of casualties from attacks across the breadth of Europe. Paris, Ankara and Istanbul have all been similarly targeted in the past five months.

But the United States has yet to see such a sophisticated ISIS attack, with trained fighters and direction from the organization's leadership.

It's not that it couldn't happen here, but military, law enforcement and other security-focused officials agree that it's much less likely. As Secretary of Defense Ash Carter told CNN's Carol Costello on Wednesday, "We don't have them (terrorists) here in the United States in anything like the numbers they do in Europe."

Why is the risk to Europe so much greater? Here are five reasons.

1. Europe has more law enforcement and intelligence gaps

Counterterrorism experts and Western officials have cited open borders and gaps in European security services, particularly Belgium's, as a major issue that allowed for the Brussels attacks to take place.

U.S. officials say the lack of intelligence-sharing among European countries, for reasons spanning from lack of political support for integration to distrust between some nations to the absence of a common E.U. defense and intelligence body, has exacerbated the problem.

The failure to coordinate was on display after Turkish President Recep Tayyip Erdoğan announced Wednesday that one of the Brussels suicide bombers had been arrested near the Syrian border and deported, and that Turkey had warned Belgian officials of his links to terrorism.

Earlier this month French Interior Minister Bernard Cazeneuve told an audience at George Washington University that "the threat has never been higher," and though he was happy with cooperation between the international and domestic French security services, he stressed the immediate need for enhanced cooperation among European governments in the security realm.

2. The U.S. doesn't have open borders close to ISIS' heartland

Open borders between E.U. countries complicate the issue, making it relatively easy to move small arms like AK-47s across Europe despite strong gun control laws in most nations on the continent. Millions of such weapons are left over from the series of armed conflicts that ravaged the Balkans following the collapse of Yugoslavia in the 1990s.

Failure to track people coming into Europe and coordinate patrols of Europe's external borders is also a major issue. German Interior Minister Thomas de Maizière told national broadcaster ARD Thursday that "there are too many gaps" in Europe's "external borders." Maizeire has called for the creation of a pan-European database to track visitors.

Additionally, once inside Europe, terrorists have been able to exploit open borders between E.U. member states. The Paris attackers crossed into France from Belgium. And weapons used in the Charlie Hebdo attack in January 2015 in Paris were also smuggled across the border.

Furthermore, Turkey shares a 500-mile-long border with Syria, and while efforts have been made to tighten that border, ISIS still controls significant territory on the Syrian side. It is possible to drive from Syria to Germany in about 30 hours according to Google maps.

3. Europe's politics can work against catching terrorists

Civil liberties and privacy debates calling for the limiting of surveillance and other tracking tools on individuals under suspicion further raises questions about the security services' ability to head off terror attacks and catch perpetrators.

In Belgium, the divided nature of the government and its population means institutions often work at cross-purposes.

Frank Cilluffo, director of the Center for Cyber and Homeland Security at George Washington University, described Belgium's law enforcement structure as "fragmented, some would even say Balkanized."

He told CNN that the Belgian system was split between federal and local law enforcement units that rarely cooperated effectively, so it is "hard to know who is in charge and who is in charge of what."

He said that there were plenty of warning signs prior to Tuesday's attack and the fact that suspected perpetrator Salah Abdeslam managed to hide out in the neighborhood of Molenbeek "was head-scratching ... You literally had the world's most wanted man hiding in plain sight."

The situation is compounded by Belgium's heterogeneous make-up. Dutch-speaking Flanders in the north, French-speaking Wallonia in the south and the Brussels capital region in the center have differences in language that can trip up law enforcement cooperation as do Belgian politics: From 2010-2011, Belgium took 589 days to form a government. In fact, Belgian Interior Minister Jan Jambon's political party once favored Flanders' outright independence from Belgium.

4. The U.S. has fewer homegrown ISIS fighters

The number of Western Europeans who have traveled to Iraq and Syria to fight alongside ISIS and other extremist groups dwarfs the number from the U.S. who have done so.

Approximately 100 Americans have joined the ranks of ISIS and other militant groups, according to the International Centre for the Study of Radicalization and Political Violence. It notes that 4,000 Western Europeans have ventured to the region as foreign fighters. This includes some 1,800 from France, 500-600 from the U.K., 500 Belgians and 200 from the Netherlands.

For every one million Belgians, about 40 have joined extremist groups in Iraq and Syria. This is by far the highest per capita rate of foreign fighter participation.

There are also growing concerns that these foreign fighters have infiltrated the hundreds of thousands of Syrian refugees that are attempting to gain asylum in Europe.

The military commander of NATO, Gen. Philip Breedlove, told Congress this month that the mass influx of migrants was allowing ISIS to spread "like a cancer, taking advantage of paths of least resistance and threatening European nations, and our own, with terrorist attacks."

Cilluffo told CNN that the training in small arms, bomb-making, secret communications and weapons handling obtained while in the region make these foreign fighters much more dangerous and harder to detect.

"They learned battlefield skills, they made contacts there, they wanted to go home, they have this ideological grievance against their own people, their own governments and the things we believe in," Carter said.

5. American Muslims are more integrated

The countries of Austria, Belgium, Bulgaria, France, Germany, the Netherlands, Sweden Switzerland, and the U.K. all had substantial Muslim minorities ranging from 5-10% of the total population in 2010, according to the Pew Research Center. This share of the population is bound to grow with the recent influx of millions of refugees fleeing violence and poverty in North Africa and the Middle East.

The U.S. Muslim population, in contrast, is about 1%. And that smaller group is more integrated and prosperous within the society.

A 2011 Pew study found that 74% of Muslim Americans "endorsed the idea that most people can get ahead if they are willing to work hard." A 2007 Pew survey found that Muslims in America were only 2% more likely to be lower-income than other Americans, while in Europe the numbers were in the double-digits. And most in Europe were more likely to think of themselves as Muslims first compared more than half who saw themselves as American first.

Many European Muslims live close together in neighborhoods like Belgium's Molenbeek, a working-class district that has found notoriety as a hotbed of violent jihadist ideology.

Molenbeek has a large, predominantly Muslim population of first-, second- and third-generation immigrants from North Africa. The neighborhood also suffers from soaring youth unemployment estimated at more than 40%.

Klugman agreed that marginalization of European Muslims in Belgium and France was a concern and that "long-term" strategies for "better integration" were needed.

He said areas where people "with same origins and a lot of problems" were concentrated needed to be broken up.

Carter also said that neighborhoods like Molenbeek complicated law enforcement efforts, saying that the attackers "were living among their families and their neighbors so it's very hard for the Belgian law enforcement, European law enforcement to find these people."

He added that some of these European citizens were "disaffected, looking for whatever reason for some cause and they find that" in ISIS.

"There's an underlying problem here, particularly in the European countries, they're going to need to confront that," he said.