Counterterrorism officials estimate that Zazi is one of 100 to 150 westerners who have gone to the Pakistan border region for terror training in the last year. Their ability to filter in and out of the isolated camps has fueled fears that “sleeper” operatives bearing U.S. or western passports are traveling back and forth with ease to train and plot attacks destined within America’s borders.

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“All you need is a shack or a house to learn how to fabricate explosives using homemade or commercially available ingredients,” said Bruce Hoffman, a counterterrorism expert at Georgetown University and a longtime government adviser.

Hoffman adds that in these harder-to-find camps, “they’re not training insurgents, they’re training terrorists for deployment to the west … Some of them may be deployed in the insurgency, but I think its obvious that their value to these groups is not fighting on the battlefield in South Asia but in being deployed back to their home or adopted countries as sleepers.”

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