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When the World Trade Center and the Pentagon were attacked on 11 September 2001, the immediate perception of the American public was that the intelligence community had let them down. To an extent, this assessment was simplistic and unfair. Because intelligence, and therefore intelligence-based warnings, are inherently ambiguous, security premised on the threat assessments of intelligence agencies had no prospect of being sufficient protection against all terrorist operations. Accordingly, 11 September constituted a security failure writ large, involving not only intelligence inadequacies but also under-appreciated vulnerabilities in a wide range of areas including immigration, law enforcement and aviation security. At the same time, it became clear that the two most important federal agencies for gathering intelligence on transnational terrorist threats outside and inside the US – the CIA and the Federal Bureau of Investigation (FBI) – had, for both systemic and circumstantial reasons, failed to apprehend the saliency of al-Qaeda’s threat. They also possessed or had access to disparate information before 11 September which could have prompted better preparedness had it been pooled, brought to the attention of high-level policymakers, or both.1