A great number they don’t know about?
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U.S. intelligence officials declined to say Wednesday exactly how many Americans who went overseas to fight with foreign militant groups have returned to U.S. soil to pose a terror threat — but acknowledged difficulties in identifying them and noted some have likely returned undetected.

“We know what we know, but we understand there is intelligence we don’t have,” Nicholas Rasmussen, the director of the National Counterterrorism Center, testified on Capitol Hill. “But it’s possible there are great numbers.”

He was joined before the GOP-led House Committee on Homeland Security by Michael Steinbach, an FBI counterterrorism expert, who told members: “This is a number we don’t know about,” regardless of the best intelligence.