There could be as many as 30,000 foreign-born members of militant group Islamic State that could soon be returning to their home countries ready for radicalization and terrorism, according to the latest research.
European cities remain on high alert following the Paris attacks in November, in which 130 people died after European-born terrorists carried out bomb and gun attacks, and the lockdown of Brussels. And there are fears of further attacks.
Security services in Europe remain on high alert should members of Islamist terrorist groups, such as the so-called Islamic State, strike again in revenge for the Western airstrikes against ISIS in Syria and Iraq.
"The risks presented by returning jihadis will likely only increase over the coming year, as more of the estimated 30,000 foreign fighters who have joined Islamic State are expected to return to their countries of origin," analysis firm Verisk Maplecroft said in a report released in early December.
"Islamic State has illustrated its adeptness at reaching out to predominantly young Muslims across Europe, Africa and Asia, as well as the Middle East and North Africa, through an extensive propaganda apparatus."
ISIS' use of social media and slick productions has enabled the group to boost its "virtual community of would-be jihadis around the world," Verisk noted.
With the group able to attract foreign recruits online, ISIS has been inciting its followers to commit acts of terror at home or so-called "lone wolf" attacks. It is estimated that ISIS has attracted around 30,000 foreign fighters from as many as 100 countries since 2011, according to the Institute for Economics and Peace.
The institute's 2015 "Global Terrorism Index", published in November, shows that many of those foreign fighters came from other Muslim nations such as Tunisia, Saudi Arabia, Jordan, Morocco and Turkey (the "top five" countries of origin).
However, rivalling the numbers of fighters coming from across the Muslim world were thousands of fighters from the West. As many as 2,700 were estimated to be from Russia, as many as 1,750 from France and hundreds from Germany, the U.K. and Belgium (the top five non-Muslim countries of origin for ISIS fighters).
This is reflected in what happened in Belgium and France this year: The majority of the Paris attackers were born and bred in France and Belgium, the second generation of migrants.
Europe therefore is waking up to an uncomfortable reality: Unemployment and disenfranchisement, and a lack of integration, can become a combustible mix for some young men and women, making them easy to radicalize against the West with many journeying to Syria to take up arms for ISIS – or to stay and fight on home soil.
Indeed, ISIS has encouraged its followers to commit attacks at home saying that those who could not journey to Syria should attack within the so-called "crusader" nations – those involved in airstrikes against ISIS, such as the U.S., France, Russia and U.K.
That advice has been borne out in attacks directed at Europe at home and abroad. From the Charlie Hebdo massacre in Paris at the start of the year to the attack on British tourists in Tunisia, the downing of a Russian jet in Egypt in October and the Paris attacks that followed in November, those responsible for carrying out these predominant attacks have all claimed to be ISIS members or affiliates allied to the group.
Paul Beaver, a U.K.-based defense analyst, told CNBC that, Western European capitals were "bracing themselves for another year of high threat levels" as intelligence and security services work to ensure that "home-grown terrorists" with ISIS allegiances do not create another Paris-style attack.
"The risk is that the terrorists will be become more innovative as they try to find new ways of attacking and damaging the Western way of life. Link this aspiration of some form of pay back from the damage inflicted on so-called IS in Iraq with a desire for a caliphate from Spain to Indonesia and the result is increased pressure," he added.
Part of the problem, Beaver noted, was that unlike terrorist groups that operated in the 1980s and 1990s, such as the Provisional IRA in Northern Ireland, "the new brand of terrorist does not require any form of central control" and are predominantly individuals without 'cells' in the traditional sense.
As Europe faced a new kind of terrorist, people could have to relinquish some closely held freedoms as authorities looked to strengthen their response to the threat although there are concerns that this plays into the terrorists' hands.
There are also fears that pointing the finger at foreign-born fighters will play to rightwing parties in Europe, creating more divisions and anti-immigration sentiment at a time when there are thousands of refugees fleeing civil war in the Middle East. Europe as a whole faced some uncomfortable decisions, Beaver said.
"The security services need more and more resources and capabilities to monitor borders, passports, telecommunications and all the other attributes of a modern, civilised society that a liberal democracy holds dear. There will be more intrusion into private lives; there will be greater risk; there will be another terrorist outrage; but there will be giving in to demands which the vast majority of people in Europe find repulsive," he noted.