Responding to a call of a mass shooting, police officers in the United States are trained, above all else, to stop the gunman. Act with urgency. Defend innocent lives.

As new questions emerged Friday about the police response to the shooting at Robb Elementary School in Uvalde, Texas, experts described those principles as the central tenets for handling such circumstances — a set of protocols that have evolved significantly over the past two decades but are widely accepted by law enforcement agencies in the United States.

Officers are taught to enter quickly in small formations — or even enter with one or two officers — to disable any gunman. Texas protocols, included in materials that Uvalde officers were trained on as recently as two months ago, advise that an "officer’s first priority is to move in and confront the attacker. This may include bypassing the injured and not responding to cries for help from children.”