Can U.S., Europe curb threat of Western fighters returning from Syria?

With thousands of Westerners, including dozens of Americans, traveling to Syria to fight in the brutal civil war there, U.S. and European officials are grappling with how to quell the threat of those fighters returning home to commit more violence.

"This is a global crisis in need of a global solution. The Syrian conflict has turned that region into a cradle of violent extremism," U.S. Attorney General Eric Holder said in a speech for Norwegian diplomats in Oslo this week. "But the world cannot simply sit back and let it become a training ground from which our nationals can return and launch attacks. And we will not."

U.S. intelligence officials estimate that there are more than 7,000 foreign fighters in Syria from more than 50 countries. The notion of foreign fighters is nothing new -- a declassified State Department report from 1993 notes that "the Soviet invasion of Afghanistan reverberated throughout the Muslim world, summoning thousands of volunteers to fight the jihad."

But thanks to the nature of the Syrian war and modern mass communications, the involvement of foreign fighters in this conflict is larger than it has been in just about any other instance, Matt Levitt, a fellow at The Washington Institute for Near East Policy, told CBS News.

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"It is the latest incarnation of an old phenomenon, but it is the largest in scale," said Levitt, who previously served as the deputy assistant secretary for intelligence and analysis at the Treasury Department and before that as an FBI counter-terrorism analyst.

"You're dealing with a conflict that is happening at a time when mass communication exists, in a place that's not off in Afghanistan, but in Syria -- off the back step of Europe," he said. "It's closer in a very physical and virtual way."

The conflict is drawing people in, he said, because they see the crisis unfold in real time and become convinced the rest of the world isn't doing enough to help. And while committing jihad in the West may seem like a bridge too far for Westerners, they may travel to Syria in the name of defending their fellow Muslims.

"The sectarian nature of this conflict brings it to a whole new level and is one of the forces that makes this so magnetic, such a draw," Levitt said. "It's closer, it's easier [to get to], it's sectarian in nature, it's defensive -- put all these things together, and it's just a bad mix."

Most foreign fighters end up dying on the battlefield, but those who want to return are difficult to track. Those from the West can travel back to Europe or potentially the U.S. visa-free. That problem is aggravated by the fact that fighters in Syria could join forces with terrorists with sophisticated bomb-making techniques, like Ibrahim Al-Asiri.

"Authorities are worried about continued bomb-making evolutions and attempts to evade security procedures," CBS News Senior National Security Analyst Juan Zarate said. "We know that bomb making experts like Ibrahim Al-Asiri -- the member of al Qaeda in the Arabian Peninsula in Yemen -- is still out there, still innovating, still training others."

The U.S. has responded by stepping up security some overseas airports with direct flights to the U.S. To help quell the violence in Syria, the Treasury Department on Wednesday sanctioned three companies suspected of aiding the Syrian government. Since the onset of unrest in Syria, the U.S. has imposed sanctions on nearly 200 individuals and entities, including the government of Syria, its Central Bank, and affiliated oil companies.

Levitt called the sanctions "praiseworthy action that can make a difference on the periphery."

"They are most effectively -- arguably only effective -- if done in tandem with other tools," he added.

While no one in the U.S. is considering putting boots on the ground, Levitt said the U.S. may eventually have to reconsider air strikes in Syria. On top of that, he said, "The arming and training of moderate opposition is a long time in coming."

The administration did recently ask Congress for $500 million -- more than expected -- to train and arm vetted Syrian rebels. However, the delay in that effort allowed radical groups -- like the Islamic State of Iraq and Syria (ISIS) -- to grow and thrive, and subsequently fuel the ongoing crisis in Iraq.

Engaging in Syria is a challenge, Zarate said, in part because U.S. intelligence there is "deficient in many ways."

"We don't have the kind of visibility we need into all elements of this network," he said, which is "now diverse in terms of nationalities involved."

As the al Qaeda-driven movement metastasizes, he said, "The question is are they operating singly and on their own, or are they beginning to share techniques and modalities... This is a tangled web where the outer rims of the network are beginning to interact with each other, and that's very dangerous."

While the U.S. is considering its options, the Europeans are "a little apoplectic about how quickly this has become so serious an issue for them," Levitt said. The threat became clear for many Europeans after a Frenchman, suspected to have spent time fighting in Syria, was linked to a shooting that occurred in May at the Jewish Museum in Brussels.

On Wednesday, French lawmakers unveiled a bill to ban foreign travel of those suspected of being radicalized, Agence France-Presse reports. Additionally, AFP reported, nine European Union ministers agreed in a closed-door meeting Tuesday to a plan to identify more young people who intend to fight in Syria.