What happens when Western fighters return from Syria?

Attorney General Eric Holder said dozens of American citizens are believed to be fighting in Syria, along with thousands of Europeans.

The top official from the Netherlands told CBS News this is quickly becoming everyone's problem.

"If you think that in the United States you have a problem with a threat, so you're bringing the airport security and the airline security up to the next level, then usually you will see that in Europe we do the same. Because you know, your threat is our threat, and the other way round," said Rob Bertholee, head of the Dutch General Intelligence and Security Service.

"Your terrorist is my terrorist. You know, there is no such thing as a Dutch terrorist cell, 'That's your problem.' No," he said.

In a rare interview, Bertholee told CBS News that European intelligence agencies are alarmed by the spike in the number of Westerners going to fight jihad in Syria - and the risk of them returning home to carry out attacks.

The concern is that after the violence of Syria - including witnessing beheadings and other atrocities--Western fighters return home, a ticking time bomb in their communities.

"When they come back, be it battle-hardened or traumatized, or a combination of those, they might become active as an attacker, as a terrorist," Bertholee said. "They could become active tomorrow, next week, but it could also take four or five years."

That makes the job of monitoring the jihadis all the harder.

Social media is another problem. It has changed the way they communicate and operate.

"You know they all fly in a swarm, there's no leader there. There's nobody who says, yeah, we have a map and we have to go this way. But amazingly, they all go the same way. That is what we see," said Bertholee.

How much more difficult does that make the job of Bertholee and other intelligence agency chiefs?

"Well, you know, if you can single out a leader then you know where to attack, where to move, where to focus your attention," he said. "If you don't have a leader, if you have a swarm that is self-guiding and self-deciding, it's much harder to focus your attention, your efforts."

Following these fighters once they return will be a lot tougher than it sounds because there is free passage of movement between many countries in Europe.

And it's worth pointing out that the United States doesn't require visas for most European citizens to travel to the U.S., so the risk is certainly not limited to Europe.