Since 2001, the bureau—often helped along by informants—has been instrumental in stopping at least 40 known terrorist plots, most of them smaller, “lone-wolf” schemes. Although it has faced some criticism for its activities and investigative techniques, the bureau’s post-9/11 record is remarkable, with no subsequent Al Qaeda attacks on U.S. soil. The person who came closest to breaking that streak, according to federal prosecutors, is Najibullah Zazi.

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