America built a fortress after 9/11. Then the threat started hiding in plain sight

Twenty-five years after the September 11 attacks, law enforcement officials have identified lone actors radicalized through online platforms as a primary terrorism threat to the United States, according to Department of Homeland Security assessments. The shift reflects a change in attack methodology from the coordinated operations that characterized al-Qaeda's pre-2001 planning. Federal agencies have adjusted counterterrorism resources and surveillance capabilities to address distributed threats rather than centralized organizational structures. The number of arrests related to online radicalization has increased annually since 2015, according to data from the FBI's counterterrorism division.

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