During an active shooting situation, American law enforcement officers are taught that their response should focus on two principles: first “stop the killing,” and then “stop the dying,” according to a training program based in Texas that is considered the national standard. The response should center on neutralizing the gunman, the program says, and then on getting medical aid to anyone who has been injured…

Officers are taught to enter quickly in small formations — or even with only one or two officers — and act to contain and neutralize any gunman. “DO NOT waste valuable time searching areas where you know there is no violence occurring,” officers are told in a training bulletin from the Louisville Metro Police Department. “Go straight to the source of the violence.”…

If the gunfire stops, the situation may change to a barricade or hostage situation, which calls for a different, slower approach, experts say. The priority becomes making contact with the aggressor and starting negotiations. Although hostage situations can require complex judgment calls — particularly if trapped victims are wounded and need treatment — law enforcement experts say negotiating has repeatedly saved lives.