LONDON (Reuters) - The head of Europe’s police organization Europol said on Tuesday the continent was facing its greatest security threat in more than a decade, with as many as 5,000 Europeans who have joined fighting in Syria posing a risk to their homelands.

Europol Director Rob Wainwright also echoed warnings from spy chiefs and some political leaders in the wake of last week’s deadly attacks by Islamist militants in Paris that European security agencies faced a “capability gap” which could leave their countries at risk.

“It is certainly the most serious terrorist threat Europe has faced since 9/11,” Wainwright told a British parliamentary committee, referring to the Sept. 11, 2001 attacks in the United States.

Many European countries are on high alert following last week’s attack by Islamist gunmen on a satirical journal and a kosher supermarket in Paris which killed 17, with France planning to deploy 10,000 soldiers.

European security officials have long warned that radicalized fighters returning from Syria pose a threat and Wainright said 3,000-5,000 EU nationals could come back with the intent to carry out operations similar to the Paris attacks.

He told British lawmakers his agency had collated a database of 2,500 suspects.

Wainwright also warned of a risk of sleeper cells, noting that the two attackers who carried out the shootings at the Charlie Hebdo weekly, brothers Cherif and Said Kouachi had traveled to Yemen in 2011 for training.

“The problem we are dealing these days is not just about Syria and Iraq it is about other terrorist networks around the world, in Africa, in the Arab peninsula for example, that have franchise movements of the al Qaeda brand,” he said.

He said Europol had passed “60 urgent intelligence leads” to French police following the attacks.

British Prime Minister David Cameron has promised to give security and intelligence services powers to monitor Internet communications, responding to calls from spy chiefs.

Wainright said highly encrypted online communications were for now effectively out of reach of law enforcement agencies. “The reality is today the security authorities don’t have the necessary capability, I think, to fully protect society from these kind of threats,” he said.