The head of the European police agency Europol, Rob Wainwright, said yesterday that up to 5,000 European nationals have joined the ranks of jihadist militants.
"We're talking about 3,000-5,000 EU nationals," Wainwright told a parliamentary committee when asked how many foreign fighters had left from Europe.
"We're dealing with a large body of mainly young men who have the potential to come back and have the potential or intent and capability to carry out attacks we have seen in Paris in the last week," he added.
Wainwright called for greater control and censorship on jihadi groups' use of social networking sites.
"We have to have a closer, much more productive relationship between law enforcement and technology firms," he said. "Social media is a recruitment tool. Not just that, social media is a propaganda tool."