Which tragic event prompted police agencies across the US to alter their tactics?

The Columbine High School shooting in 1999 was a pivotal moment that significantly influenced police tactics nationwide. Before this event, law enforcement protocols for responding to active shooter situations often involved securing the area and waiting for specialized units to arrive. However, the tragic and fast-paced nature of the Columbine incident exposed the need for a more immediate and aggressive response to prevent further loss of life.

In the aftermath, police agencies began to adopt a strategy known as "immediate action" or "active shooter" response tactics. This approach emphasizes the importance of officers engaging the threat as rapidly as possible, rather than waiting for backup to arrive. This shift has had lasting implications for police training and operational protocols in similar situations, making it a crucial event in the evolution of law enforcement responses to active shooter incidents.

The other events listed, while tragic in their own right, did not lead to such a fundamental change in police response tactics regarding active shooters in schools and public venues as the Columbine shooting did.