The European Union's police organisation says the threat of terrorism across the 28-nation bloc is "acute and diverse," despite a fall in the number of attacks last year.
In an annual report on terrorism threats and trends published today, Europol says radicals who travel to fight alongside militants in conflicts like the Syrian civil war are "posing an increased threat to all EU member states on their return."
In the wake of Syria's war, "the threat to the EU is likely to increase exponentially," the report warns.
It lists seven people killed by terrorists in the EU in 2013, including a British soldier hacked to death in London by two al-Qaeda-inspired extremists.
The report is compiled annually based on statistics from EU member state police forces.
