“Of course it was not the right decision,” Mr. McCraw said. “It was a wrong decision, period. There’s no excuse for that.”

He continued, “When there’s an active shooter, the rules change.”

The best practices for such shootings have evolved considerably since 1999, when 12 students and one teacher were killed at Columbine High School in Colorado, and officers were trained to maintain a perimeter and wait for a tactical team.

“Columbine changed everything because they realized that although it was not a bad plan to wait, people will get killed while you’re waiting,” said Robert J. Louden, a professor emeritus of criminal justice and homeland security at Georgian Court University in New Jersey.

Since then, the police have increasingly emphasized speed. In an Elkhart, Ind., supermarket in 2014, officials said that a gunman who had shot two people was aiming his weapon at a third when officers fatally shot the gunman within a minute of arriving.

Other shooting massacres, too, have revealed how quickly lives can be lost. At Sandy Hook Elementary School in Newtown, Conn., in 2012, 26 people were killed, including 20 children, in six minutes before the police arrived. In Las Vegas in 2017, 59 people died at an outdoor concert festival over 12 minutes before the police closed in on the gunman’s hotel room.

In some cases, experts said, mass shooting events can transition between active situations and barricaded or hostage situations. In the latter, the priority becomes making contact with the aggressor and starting negotiations to persuade a gunman to surrender or just gain valuable time while a tactical team is assembled.

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