Terrorism experts Dr. Walid Phares and Peter Bergen

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Peter disagrees with Dr. Phares about the danger posed by a "sleeper cell" of native-born Americans or recent immigrant terrorists. Instead, he points to the dangers lurking in Europe.

"I'm concerned about people getting radicalized in Europe and coming over here," he says. "We had an amazing case of a female suicide bomber in Iraq in November. She was Belgian. She and her husband went to conduct a suicide operation in Iraq, but they could have just as easily come here to try and conduct some kind of operation here. They were traveling on European passports, and they didn't have criminal records. I think in the future you can imagine an attack conducted in the United States by somebody who is a woman."