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The rapid collapse of a U.S. diplomatic compound in Libya exposed the vulnerabilities of State Department facilities overseas. But the CIA's ability to fend off a second attack that same night provided a glimpse of a key element in the agency's defensive arsenal: a secret security force created after the Sept. 11, 2001, attacks.

Two Americans killed in the Libyan city of Benghazi were members of the CIA's Global Response Staff, an innocuously named organization that has recruited hundreds of former U.S. special forces operatives to serve as armed guards for the agency's spies.