This development comes as European officials told CNN they believe ISIS is ratcheting up its planning for international attacks to retaliate for losses in Syria, Iraq and Libya.
An exclusive CNN report this week showed the Paris slaughter, which killed 130 people, was meant to be broader and include other countries, such as the Netherlands. CNN also reported information from the Belgian prosecutor that a man allegedly linked to the Paris attackers was on the loose for months after authorities learned of his existence.
"The outlook is as bleak as it has ever been," said Paul Cruickshank, editor of CTC Sentinel, a publication issued by the Combating Terrorism Center at West Point, and a terrorism analyst for CNN. Characterizing the mood at a recent high-level European conference on counter-terrorism, Cruickshank said, "We're talking beyond severe concern."
The European countries most firmly in the crosshairs are launching air strikes against ISIS with the US-led coalition: France, Belgium, the Netherlands, the UK and Denmark, as well as Germany, which is flying target reconnaissance sorties.
US vulnerable, too
Terror analysts warn that despite the buffer provided by the Atlantic Ocean, the US is vulnerable, too.
"It is by no means immune," warned Matthew Henman, editor of IHS Jane's Terrorism and Insurgency Center in Houndsditch, UK.
He pointed to the US border with Mexico, which "is far from secure or impenetrable."
Airports are another point of entry. Western European passport holders can travel to the US without a visa. US nationals have also been able travel to Western Europe and from there to Syria via Turkey, returning via the same routes.
The difficulties European security services have had amidst these challenges illustrate how daunting it is to track ISIS sympathizers and former fighters.
First, ISIS' operating structure means that nabbing one suspected terrorist doesn't necessarily make it easier to find the others. One official likened the ISIS' structure to a series of concentric circles in which only the inner circle knows everything. People in each outward circle have a little bit less knowledge or a very specific role to play.
Someone tapped to supply weapons for the Paris attack might have had no idea who would get them or how they would be used, the official said.
Then the group's use of encrypted communications, detailed in CNN's initial report, makes it extremely difficult for security services to locate and identify individuals.
"Even when they locate suspected ISIS operatives, security services are finding that pretty much the only way to track their communications is to bug their homes or cars," Cruickshank said.
The CNN investigation showed yet another layer of protection for ISIS employs, insisting that operatives use pseudonyms, leaving them unable to give authorities their co-conspirators real names.
Europe bracing for ISIS presence to grow
European officials are now concerned that ISIS' European presence is set to grow. As ISIS suffers losses in Syria, Iraq and Libya, these officials are bracing for the group's fighters to return to Europe in greater numbers, not only to launch attacks, but also to mobilize others, sources told CNN.
Cruickshank said official estimates of the number of European passport holders who have traveled to Syria and Iraq range from 6,000 to 9,000 people, many of whom have traveled to fight for ISIS.
Europe's tightening security net could prompt the group to "send as many fighters back to Europe as possible in the awareness that its ability to do so would not last indefinitely," said Henman of IHS Jane's.
Against this backdrop, some European officials said security services are struggling.
Hazim Fouad, an analyst who focuses on Islamic extremism for the Bremen branch of Germany's Office of the Protection of the Constitution, told the Combating Terrorism Center in July that "during the past few years a wave of foreign fighters, a general increase in the Salafi movement, as well as a sky-rocketing of right-wing-motivated criminal offenses, stretched security agencies up to and beyond their limits."
Henman noted that security services across Western Europe, let alone in less wealthy Central Europe and the Balkans, are operating at full capacity just trying to identify all those returning from Syria and Iraq and then determine who poses a genuine threat.
"Budget and resource constraints, alongside sub-optimal intelligence-sharing practices and protocols between countries, only hamper these efforts further," Henman said.
That may be why it took European security officials months to track down a man they suspected of being tied to the Paris attackers. Abid Tabaouni, named publicly for the first time in the exclusive CNN report, was the subject of a Europe-wide arrest warrant that Austrian officials requested in February.
He wasn't brought into custody until July, when Belgian security officials located him in Brussels. European investigators are now examining an apparent visit Tabaouni made to Amsterdam while security agencies were searching for him, sources told CNN.
A CNN analysis of his Facebook account revealed that on February 24, 2016, Tabaouni posted a picture of himself in front of Amsterdam's waterfront skyline and that he reposted the same picture on March 26.
Investigators are aware of these posts and are looking into whether they have any significance, sources told CNN.
Analysts warn that ISIS propaganda has focused overwhelmingly on inciting individuals -- so-called lone wolves -- to launch attacks independently.
"It is not just an additional threat that the West now faces, but an additional element for the security services to try and contend with," said Henman, who pointed to the December 2015 assault on a regional center in San Bernardino, California, and the June strike on a nightclub in Orlando, Florida.
Repeats of those attacks "will very much be what the Islamic State tries to inspire and provoke in the coming months," he said.