Americans and Europeans traveling to the Middle East to join Islamic State (ISIS) terrorists pose the "No. 1 threat" to the United States, said former Department of Homeland Security Secretary Janet Napolitano.
Napolitano told MSNBC's "Morning Joe" the difficulty in tracking their travel makes it hard to determine which Americans or Europeans might enter the United States to conduct terror activities.
"The No. 1 threat to the homeland is Americans or Europeans who have passports who have been over there, and who are trying to come back, who may try do something in the United States," Napolitano said Monday.
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Napolitano served as Secretary of Homeland Security under President Barack Obama, leaving in 2013, and now serves as president of the University of California. She said U.S. capabilities do not offer a "100 percent fix" in following the estimated 100 Americans and thousands of Europeans who have joined ISIS.
"Obviously, eyes and ears are attuned to Americans who may be traveling into the region, trying to track travel with the travel documents that DHS does collect. But it's a very big region, and these are very big borders. And, freedom of travel is freedom of travel," she said.
The Islamic State was "on everybody's radar screen" when she was Homeland Security secretary, Napolitano said, but said they weren't "the dominant source of conversation."
"They've grown exponentially, in terms of size and capacity and capability," she said. "There were lots of discussions with how do you get the region and the countries in the region to participate in a more vital and a more vigorous manner."
Bill Kristol, editor of The Weekly Standard, told "Morning Joe" that the "growth of ISIS is unbelievably bad," and maintained the terrorist organization "should never have been allowed to get this far." He said he hopes Obama now understands the severity of the threat of ISIS.
"I very much hope that he now grasps reality, accepts reality, and acts decisively," Kristol said Monday.
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