Up to 5,000 European Union citizens have joined jihadist militant ranks, the head of European police agency Europol told British lawmakers on Tuesday.
"We're talking about 3,000, 5,000 EU nationals," Rob Wainwright told a parliamentary committee when asked how many foreign fighters had left from Europe.
The Europol chief added that the mostly young men pose a threat to their country of origin if they return there.
"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," Wainwright said.
Calling the returning militants "the most serious terrorist threat that Europe has faced since 9/11," Wainwright called for greater scrutiny of the use of social media, which he said was used as a recruitment tool by jihadists.
"We have to have a closer, much more productive relationship between law enforcement and technology firms.
"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," he added.
The names of about 2,500 suspects have been collected by Europol from the security services of various EU countries, Wainwright said.
The European Union's counter-terrorism chief Gilles de Kerchove estimated in September 2014 that around 3,000 European citizens had joined jihadists in Syria and Iraq.
De Kerchove said in an interview with AFP in Brussels on Tuesday that around 30 percent have returned to their EU countries.
The issue of home grown jihadists has gained prominence in recent days following a string of attacks in Paris by assailants born in France.
Said Kouachi, one of two brothers responsible for an attack on the French satirical magazine Charlie Hebdo last week, studied at a Yemen college founded by a fundamentalist cleric before undertaking weapons training with an Al-Qaida affiliate.
His brother Cherif was part of the so-called Buttes Chaumont network in Paris, which helped send recruits to join the Iraqi branch of Al-Qaida.