Top U.S. and British counterterrorism officials said Tuesday that the growing number and variety of foreign fighters streaming into Syria is unprecedented in recent history.

“The rate of travel into Syria [by foreign fighters] is greater than we saw into Afghanistan prior to 9/11,” Randy Blake, a senior strategic advisor in the U.S. Office of Director of National Intelligence, said Tuesday during a panel at the annual International Association of Chiefs of Police (IACP) conference in Orlando, Florida. “It’s greater than anything we’ve seen into Afghanistan, into Yemen, into Somalia, into Iraq, or anything that we’ve seen in the last 10-year period.”

Blake said the number of Westerners heading to Syria to fight has risen so rapidly in recent weeks that a new law enforcement video shown at the four-day event is already out of date.

“The video said there are somewhere around 12,000 foreign fighters in Syria. We would update that number to about 16,000 foreign fighters who have traveled to Syria from over 80 countries,” Blake said, adding that roughly 2,000 of those fighters hail from Western countries -- including "at least 500 from the U.K, 700 from France, 400 from Germany, and more than 100 Americans [who] have traveled, or tried to travel into Syria.”

What’s more, authorities have found it nearly impossible to create a demographic profile of these Western volunteers.

“This particular conflict zone and this particular foreign fighter threat runs the gamut in terms of age,” said John Adams, the FBI’s deputy director for counterterrorism.

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“We’re currently age-ranged from minors of 15 years old, which is our youngest traveler, up to 63 years old, which has been our oldest traveler. It’s very, very difficult to try and identify a particular age group that this particular foreign fighter message resonates with,” Adams said.

No one knows for sure what is motivating so many Westerners to head to Syria, but one U.S. official offered a personal theory.

“When we try and figure out: what is different about this foreign fighter foe? What separates this conflict from Afghanistan, from Somalia, from Bosnia?” asked Andrew McCabe, assistant director in charge of the FBI’s Washington field office.

“To me, the instant appeal of participating in the caliphate that’s happening now, I think that resonates with our young people. “

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“Our culture is one that grew up with instant gratification, instant information,” said McCabe. “Al Qaeda’s message has historically been, ‘This is a long war, we’ll fight the Great Satan [and] we’ll try and establish a caliphate sometime in the future.’ ISIL has come forward and said, 'The time is now, it's happening now. We have our own territory, come join us.'”

“And I think that’s one of the reasons [ISIL] has such magnetism for this at-risk population.”

John Noble, a veteran counterterrorism expert posted to the British Embassy in Washington, D.C, said he is most concerned with what happens when these fighters come home.

"Clearly the military action underway is a really important part of mitigating the threat [of ISIL]," Noble said. "But the reality is that the threat is going to be dealt with on the streets of Europe and on the streets of the United States.”

So far, at least a dozen U.S. residents have been grabbed at airports and borders as they tried to make their way to Syria to fight on the side of the most radical elements in the civil war against Bashar al-Assad. Those who have been prosecuted have been charged with giving material support to terrorist groups and there has been no indication that any of them wanted to return to the U.S. to carry out terrorist attacks. Karen Greenberg, director of the Center on National Security at Fordham Law School, says law enforcement is concerned that once in Syria or Iraq, radical jihadis will attempt to recruit them for such missions.

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Click here to explore interactive map with details of the cases of a dozen U.S. residents -- 11 of them Americans -- accused of trying to join the fight in Iraq or Syria as well as the three Americans confirmed to have died in the fighting.

Law enforcement sources tell NBC News that several other cases working their way through the system in various jurisdictions. The cases include American citizens who have actually made their way to the battlefields of Syria.

Although many Westerners have gone to Syria to fight for groups like al-Nusra and Ahrar ash-Sham, the success of ISIS --and the establishment of the caliphate -- is now drawing fighters from those groups to the Islamic State, say U.S. intelligence officials. At least three Americans who went to Syria to join those groups were later killed while fighting on behalf of ISIS.