“Waiting an hour is disgusting,” said Sean Burke, a recently retired Lawrence, Massachusetts, school resource officer who is president of the School Safety Advocacy Council, which trains districts in how to respond to shootings. “If that turns out to be true, then it is a disgusting fact.”…

Waiting for specialized tactical units used to be standard practice in responding to shooters. That changed after the shooting at Columbine High School in Colorado in 1999, when police waited nearly an hour for a SWAT team to enter the building — during which time 12 students and one teacher were killed.

To save time and lives, police began sending in the first four or five officers to arrive. That standard changed again in recent years to emphasize that officers should do everything they can to interrupt shooters, even if they are alone and without backup.

With mass shootings, time is precious. An FBI study of 160 “active shooter” incidents in the U.S. from 2000 to 2013 found that the majority of shootings in which the duration could be determined ended in five minutes or less, with about half of those lasting no more than two minutes.