Matthew Olsen, the outgoing director of the National Counterterrorism Center, said Friday there are 12,000 foreign fighters in Syria – a significant jump from previous estimates, indicating a “greater and more complex” threat than in recent years.

Speaking at the annual Aspen Security Forum in Colorado, Olsen said there are now 5,000 more fighters on the ground than in January. Of them, more than 1,000 are from Europe and more than 100 are from the U.S., he said.

While Olsen has been warning about the threat posed by radicalized young Muslims with western passports who have traveled to Syria to fight President Bashar al-Assad’s regime for years, he said Friday the current threat was more complex than it had been. He also said that Al Qaeda’s core has identified Syria “as the place to go.”

Olsen added that “veteran” Al Qaeda individuals are in Syria to pursue operatives for external attacks.

Fox News has previously reported that the intelligence community believes operatives from Al Qaeda in Yemen are in Syria sharing their bomb-making capabilities. The group, Al Qaeda in the Arabian Peninsula or AQAP, has a master bomb-maker whose devices use non-metallic explosives that can evade traditional airport screening.

AQAP has tried three times to blow up a U.S. jet, including the failed underwear bomb plot in 2009. In that case, bomb-maker, Ibrahim al-Asiri, built an underwear bomb in an effort to down a Detroit-bound airliner. U.S. officials have said they have evidence that AQAP operatives have tested new bomb designs in Syria.

Former FBI Director Robert Mueller, who was also speaking at the conference, said the unrest in Gaza also “will feed and fuel” an increase in the number of individuals who will engage with radical groups, specifically Iraq and Syria.

The National Counterterrorism Center, the country’s terrorism information clearinghouse, was created following the Sept. 11, 2001, attacks. The agency is responsible for analyzing and integrating information gathered from intelligence agencies and providing plans and recommendations to the president and senior officials.