WASHINGTON -- Every day, terrorists trained in Syria and Iraq travel freely back to their homes in Europe and the U.S., and counterterrorism officials can't keep up with all of them.
"The more I work, the worse it gets," says Gilles de Kerchove, European Union counterterrorism coordinator.
At a recent meeting with American security officials, de Kerchove said Western intelligence agencies have no clear answer to the movements of al Qaida-linked foreign fighters and operatives from The Islamic State of Iraq and the Levant (ISIL) who number in the thousands.
International intelligence estimates indicate more than 10,000 recruits and hardened militants from all over the world have been in the region, and the danger is mushrooming.
"The threat is much more diverse than it was before. Since last year, we've seen the foreign-fighters phenomenon increasing significantly. I think we've seen more than 2,000 Europeans who've been to Iraq and Syria, and it is still growing," said de Kerchove.
Hundreds of Americans are said to be among those foreign fighters who made the journey to the region.
Some are known to have returned to the U.S.
"We need to get out and talk more about the nature of that threat, so there's a better understanding about it among the American people and among decision-makers -- and to lay out the nature of the threats and prescribe the tools that are needed to combat it," National Counterterrorism Center Director Matthew Olsen said.
Intelligence experts say the U.S. decision to launch air strikes against ISIL targets in Iraq has increased the likelihood terrorists returning from Iraq and Syria will try to attack the U.S.
"Taking on ISIL in any way is problematic because they will come back and hit us," said Robert Baer, a former CIA covert operative.
Baer added that that process may have already begun.
"Various intelligence and homeland security officials have told me that they are already in this country. Some of these people have American passports and went over to Syria and Iraq and got training and are back, and some have slipped across the Mexican border."
Baer characterizes his conversations with the officials as confidential, but that they contained serious warnings.
"They're telling me, 'This is a big threat to this country -- people coming out of the Levant, are here ready to strike.'"
Counterterrorism analysts are pouring over every scrap of intelligence they run across because ISIL leader Abu Bakr Al Baghdadi, according to a senior U.S. intelligence official, "has already stated many times his interest in attacking the U.S. and claims to have fighters in the U.K., France, Germany and the U.S."
The official said, "the transit route from Iraq to Syria to Turkey to Europe and on into the U.S." is a key concern, because Americans with travel documents can easily enter into the U.S.
Even though they can't keep tabs on every suspicious person who travels to an area frequented by terrorists, U.S. authorities have adopted effective tactics that increase their ability to stop a potential terrorist from launching an attack. The senior intelligence official says sometimes they choose to "identify the potential recruits, and sometimes stop them from traveling or simply monitor them."
The inability to close down all of the possible entry and attack points in the U.S. and Europe has led the analysts to focus on the most likely scenarios for whether, when, how and where ISIL or other terrorists would strike.
The senior U.S. intelligence official says ISIL fighters are "very good at executing military-style operations, and there's no reason to doubt that they couldn't launch an unsophisticated terror operation" like the West Gate Mall attack in Kenya.
The official said they're not seeing any ISIL plotting directly targeting the U.S., and no evidence that ISIL operatives are in the country, but wherever they are, there is concern about the organization's expertise with social media and the ability to mobilize quickly.
De Kerchove said ISIL multiplied the world's terrorism threat when it established in Iraq an alleged Islamic caliphate -- a state led by a supreme religious and political leader known as a caliph.
Now, "we'll have to see to what extent the proclamation of a caliphate will attract even more jihadists -- we don't know, but this could be a significant factor."
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