Since the Columbine High School massacre more than 20 years ago, police have been trained to quickly confront shooters in the horrific attacks that have followed.

But officers in Uvalde, Texas, took more than an hour to kill a shooter who massacred 19 children, a lapse of time that will likely be a key part of a Justice Department probe into the police response.

The rare federal review comes amid growing, agonized questions and shifting information from police. Authorities now say that several officers entered the elementary school just two minutes after alleged gunman Salvador Ramos and excha...