Sheer numbers of radicalized Muslims make Europe an easier target for terrorism, but the United States faces a real and growing threat from “sleeper cells” and lone wolves, according to security experts.

Europe has become a hotbed for radical Islam over the last two decades, as immigrants hole up in hovels outside major cities. Molenbeek is one such slum, in the outskirts of Brussels — home to the terrorists who attacked Tuesday, killing at least 30 and wounding hundreds. Militant Muslims are also in the ghettos outside Paris — the perpetrators of the massive attack that killed 130 this year were traced there. And Germany is battling a huge influx of Muslims; many Germans now fear attacks from radicalized Islamists.

The United States has no clear counterpoints to Europe, but there are cities with large Muslim populations, like Detroit, Minneapolis and New York City. Last year, the FBI probed an area of Phoenix, and reports emerged the agency was investigating reports that the area had become a hotbed for terrorist recruitment at an ISIS level.

All five presidential candidates on Tuesday weighed in on the Brussels terror attacks, but Ted Cruz went a step further, saying U.S. law enforcement should step up efforts to secure the homeland.

“We need to empower law enforcement to patrol and secure Muslim neighborhoods before they become radicalized,” Cruz said. “We know what is happening with these isolated Muslim neighborhoods in Europe. If we want to prevent it from happening here, it is going to require an empowered, visible law enforcement presence that will both identify problem spots and partner with non-radical Americans who want to protect their homes,” the Cruz campaign said in a statement.

While the United States has not experienced the level of terrorism over the past couple of years that has afflicted Europe, Kyle Shideler, director of the Threat Information Office at the Center for Security Policy, said the number of plots uncovered by law enforcement officials indicates that the threat remains high.

“We’ve been fortunate so far,” he said. “The FBI has been rolling people up in the ones and twos pretty much every week … Clearly, we do not have the problem Europe does, just in terms of the numbers being far higher.”

Ryan Mauro, a Liberty University professor and national security analyst at the Clarion Project, said European countries have seen a huge, rapid increase in Muslims coming from the epicenter of Islamic terrorism. But he said there is no shortage of potential threats in the United States, pointing to three recent studies estimating that 70 percent to 80 percent of U.S. mosques have ties to radical Islam, either from the textbooks they use or the content of the sermons that imams preach.

Terrorism expert Steven Emerson, in his 2003 book “American Jihad: The Terrorists Living Among Us,” tracked mosques with radical ties and found the cities with the most mosques with links to terrorism groups were in New York, Detroit, Houston and the Dallas-Forth Worth area.

Researchers from the University of Maryland and University of Massachusetts at Boston in 2012 identified 65 counties as terrorist “hot spots,” where six or more acts or attempted acts of terrorism had occurred from 1970 to 2008. The report, which examined all motivations for terrorism — not just Islamic terror — found the counties with the most incidents were Manhattan (343) Los Angeles (156), Miami-Dade (103), San Francisco (99), Washington, D.C. (79), Cook County, Illinois (68) and Alameda, California (64).

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Mauro and Shideler both agreed that the risk is growing in the United States. The prospect of sleeper cells has worried U.S. authorities at the highest levels for some time.

Sen. Dianne Feinstein said on CNN in January 2015 that she believes are sleeper cells “not only in France, but certainly in other countries, and yes, even in our own.” In September, the House Homeland Security Committee issued a report concluding that the “unprecedented speed at which Americans are being radicalized” was straining law enforcement resources.

[lz_table title=”Syrian Refugees” source=”Refugee Processing Center”]Fiscal year,,,Admitted to U.S.
2014,,,105
2015,,,1682
2016,,,955*
* first 5 months
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Shideler said organized terrorism networks undoubtedly are embedded in American society. He said terrorist groups like Hezbollah often engage in cigarette smuggling and other illegal activity has a means of funding operations in the Middle East but also are ready to strike within the United States should they receive such orders.

Beyond sleeper cells, Shideler said, the United States has a growing problem with “individual jihad,” one or two people carrying out terrorism on their own after becoming radicalized. The attack on a holiday party in San Bernardino, California, in December falls into this category.

“That makes them, actually, a lot more difficult to track,” he said.

Then there is an emerging “hybrid” threat that combines both elements, Shideler said. This refers to terrorists coming into the United States and recruiting like-minded peopled to the cause. Terrorists like that might receive instructions and encouragement from foreign-based organizations but not financial or material support and would have a great deal of autonomy in picking targets.

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“I think it’s coming,” he said. “We saw a little bit of that with the Boston Marathon bombing.”

Refugees are another potential source of terrorism. According to the most available statistics, the United States accepted 955 Syrian refugees between the start of fiscal year 2016 on Oct. 1 and the end of February. Consistent with President Obama’s commitment to increase assistance to Syrians fleeing civil war, that is a pace that would far exceed the 1,682 who came in fiscal year 2015 and the 105 who came the year before that.

Mauro said there are easier ways into the United States for terrorists other than posing as refugees. But he said an increase Syrian refugees could still pose a risk by increasing the pool of radicalized Muslims available for recruitment in the United States. He pointed to a 2014 survey indicating that 13 percent of people in refugee camps in Lebanon Jordan and Turkey had a favorable view of ISIS.

Zuhdi Jasser, founder of the American Islamic Forum for Democracy, said on “The Laura Ingraham Show” that many Muslims who come to American share anti-American views.

“They may not be violent and preaching jihadism, per se, but they’re swimming in that same anti-Americanism, anti-Semitic cesspool of ideology,” he said.