That disturbing scenario was laid out in a report by Europol - Europe's law enforcement agency -- released Monday.
The report, entitled "Changes in modus operandi of Islamic State terrorist attacks," paints a picture of a terror group whose methods keep evolving and whose threat is two fold: Coordinated attacks and lone wolf operatives.
"The Paris attacks, and subsequent investigation, appear to indicate a shift towards a broader strategy of [ISIS] going global, of them specifically attacking France, but also the possibly of attacks against other Member States of the EU in the near future," the report said.
The report details some of the ways ISIS is adapting. Here are some of the key findings:
Soft targets are the most vulnerable
Attacks will be primarily directed at soft targets because of the impact and mass casualties they generate, the report says.
Intelligence also suggests that ISIS has developed a command structure to plan and coordinate "special forces style" operations abroad. This could mean that more Paris-style attacks are currently being planned and prepared, the report says.
Attacks aren't always planned from within Syria
In addition to training facilities in Syria, there are also smaller scale training camps in the EU and in Balkan countries.
ISIS-inspired attacks do not necessarily have to be coordinated from Syria.
"Central command in Syria is believed to map out a general strategy, but leaves tactical freedom to local leaders to adapt their actions to circumstances on the spot," the report says. Operatives can choose their targets based on capability and resources, which leaves room for spontaneity and makes it hard for law enforcement to identify targets and suspects.
Recruits are young and not necessarily religious
Recruitment into ISIS happens quickly, without necessarily requiring a long radicalization process, the report says. The "romantic" prospect of being part of something important and exciting may also play a role in recruiting.
Peer pressure has replaced some of the religious components of recruiting, the report says.
Younger recruits are more impressionable and radicalize quicker. Less than half of all people arrested for joining ISIS or expressing an intention to do so have relevant knowledge about their religion. This makes them vulnerable to interpretations of the Koran that fit ISIS logic, the report says.
Recruiters use survival training to test recruits' fitness and ability. "Sports activities have been used for combat and interrogation resistance training," the report says.
A "significant" portion of foreign fighters were diagnosed with mental problems prior to joining the terror group. However, the report doesn't specify how they know this and what types of mental issues fighters may be suffering from. The report also says that a large portion of recruits have criminal records.
Refugees are not a threat, but...
There is no concrete evidence that terror groups (ISIS or otherwise) are using the current refugee crisis
to slip into Europe unnoticed. Instead, the report says that there's a more "real and imminent" danger that members of the refugee population will become vulnerable to radicalization once in Europe, and that they're being specifically targeted by terror recruiters.
They use encrypted communication tools
ISIS has taken advantage of the availability of secure and encrypted communication methods such as WhatsApp, Skype and Viber for communication and to procure goods and services such as weapons and fake IDs.
How they finance attacks in Europe is largely unknown
Travel costs, car rentals, safe houses and weapons require considerable sums of money, the report says. However, there is no evidence of ISIS -financing networks in the region.
Al Qaeda is still a threat
The report warns that Al Qaeda is still a "factor" in the region and a reason for the EU to focus broadly on religiously inspired groups.
A separate report
charges that Al Qaeda's Syrian affiliate, Jabhat al-Nusra, is a greater threat to the United States in the long term than is ISIS, making the United States' current single-minded focus on the latter group misguided.
Recently, al Qaeda or Taliban affiliated groups have claimed responsibility for deadly attacks in Pakistan
, Burkina Faso