The ranks of terror groups in Syria and Iraq continue to swell with foreign fighters, according to John Carlin, the assistant attorney general for national security.
"There are over 18,000 foreign fighters in the Syria-Iraq region," Carlin told CBS News. "That includes individuals from over 60 countries around the world."
The 18,000 figure is an increase from the 13,000 foreign fighters the United Nations reported in September of last year. More than 3,000 of the fighters come from Western Europe and more than 100 come from the United States, according to Carlin. The majority are coming from the Syria-Iraq region.
The attack on satirical newspaper Charlie Hebdo in Paris has created a renewed focus on the threat of radicalized fighters carrying out attacks in their home countries. One of the two men suspected in the massacre that left 12 dead was convicted in 2008 of recruiting fighters in France to go and fight in Iraq.
No definite link between the suspects has been drawn to a specific terror group, but Mike Morell, former deputy director of the CIA, said "there's a good possibility we're looking at an al Qaeda in Yemen-directed attack."
He points to Charlie Hebdo's history of publishing satirical cartoons that ridicule Islam.
"Al Qaeda has been very focused on media outlets that have lampooned the prophet Mohammed," said Morell.
In response to the attacks on Charlie Hebdo, Attorney General Eric Holder will fly to Paris on Sunday for international terror talks.
"We're really going to focus on foreign terrorist fighters and the problem of terrorism in Europe and in the U.S. and work on joint approaches to minimize the chances of an attack like this concurring again," said Carlin, who'll be in attendance at the talks.
One approach used in the U.S. has been a public campaign by the FBI aimed at curbing the number of individuals going overseas to fight. Launched in October, the campaign asks the public to help the FBI identify people who have traveled - or are planning to travel - overseas to engage in combat with terrorists.
"You find countries that agree on very little else agree that this is a problem that has to be stopped and has to stop now," said Carlin.