Why did police agencies increase cooperation with intelligence and national-security agencies after September 11?

After the September 11 attacks, police and national-security agencies increased cooperation primarily to address catastrophic failures in communication, coordination, and information-sharing that had previously hindered counterterrorism efforts.123

Before 2001, institutional "walls" often prevented law enforcement—which focused on prosecuting crimes—from sharing intelligence with national-security agencies, which prioritized hoarding information to dismantle entire networks. The 9/11 Commission identified that such failures allowed known militants to enter the United States undetected and prevented the investigation of suspicious activities, such as those of Zacarias Moussaoui.13

To rectify these issues, the government implemented several changes:

• Legislative Reform: The USA PATRIOT Act was enacted to facilitate information sharing by allowing government attorneys to disclose grand jury matters to intelligence and national-security officials when foreign intelligence was involved.4
• Institutional Consolidation: The Department of Homeland Security was created to integrate the functions of 22 federal agencies, aiming to improve coordination and protect the country from diverse threats.2
• Tactical Shifts: Recognizing that modern terrorism is global and complex, agencies moved toward integrated surveillance and intelligence-led policing, though these shifts have faced ongoing criticism regarding their impact on civil liberties.124