There are between 3,000 and 5,000 European Union nationals who pose a potential terrorist threat to Europe after travelling overseas to countries such as Syria, a law-enforcement chief has told MPs.
Rob Wainwright, director of Europol, told the Home Affairs Select Committee his organisation has so far collected 2,500 names of suspects from agencies across EU member states.
He warned that the mainly young men who have travelled overseas have the potential to return to their home countries and carry out attacks, as seen across Paris last week. Words: PA
Asked how many so-called foreign fighters had left Europe, Mr Wainwright said: "We're talking about 3,000-5,000 EU nationals.
"Clearly, we're dealing with a large body of mainly young men who have the potential to come back and have the potential or the intent and capability to carry out attacks we have seen in Paris in the last week."
Asked about the co-operation of internet companies in tackling extremist activity online, Mr Wainwright said more needed to be done.
"One of the important evolutions we're seeing right now in the current terrorist threat is the way the internet is used, clearly much more aggressively, much more imaginatively by the networks.
"Social media is a recruitment tool. Not just that, social media is a propaganda tool.
"We have to have a closer, much more productive relationship between law enforcement and technology firms.
"Also I think we need to have the right legalisation in place to allow the authorities to monitor suspected terrorist activity."
Asked by Committee chair Keith Vaz if Britons can sleep "soundly" at night, Mr Wainwright said: "The British people can be assured that here in the UK we have some of the most well developed and effective counter-terrorism arrangements in the world.
"That said, the threat is here and clearly affects the UK, at least as much as many other countries across Europe. It's a real threat."