This story is part of a KXAN series of reports called “Stop Mass Shootings,” providing context and exploring solutions surrounding gun violence in the wake of the deadly Uvalde school shooting. We want our reports to be a resource for Texans, as well as for lawmakers who are convening a month after the events in Uvalde to discuss how the state should move forward. Explore all “Stop Mass Shootings” stories by clicking here.

AUSTIN (KXAN) — To fully understand the law enforcement failure in Uvalde, you have to look to the past.

Specifically, 1999 and Columbine High School, two gunmen shot and killed 12 students and one teacher and injured more than 20 others. Lessons learned then still guide law enforcement’s response today — and should have in Uvalde.

“We came to realize that when people are being shot and killed — you can’t wait,” said former Austin Police Chief Art Acevedo, who has also led departments in Houston and Miami.

Acevedo, who is now a law enforcement consultant, said Columbine fundamentally changed how police, nationwide, train and respond to active shooters.

“I think that was the first time we finally realized this is not the answer; waiting is not the answer,” said Acevedo. “Waiting for the perfect tactical unit to show up isn’t the answer because we know the majority of the deaths will happen in the first two to three minutes. So, time is of the essence.”

During Columbine, officers waited 47 minutes to enter the school after the shooting started. Back then, police protocol called for setting up a perimeter and waiting for SWAT teams to arrive. That approach changed after Columbine. Training now calls for officers to immediately confront a gunman in order to save lives.

As part of legally required training for Texas school-based law enforcement, Uvalde CISD officers were told exactly that — “neutralize the threat even if that means one officer acting alone.”

The Texas Commission on Law Enforcement gave a blunt warning: “A first responder unwilling to place the lives of the innocent above their own safety should consider another career field.”

That is not the approach taken at Robb Elementary, according to hallway video leaked to the Austin-American Statesman. The video shows officers defying protocol and past lessons, waiting 74 minutes to confront the gunman — about 30 minutes longer than it took officers at Columbine 23 years ago.

“It was hard to watch officers stand in the hallway, keeping themselves safe,” said Acevedo. “Not doing what’s required of us — which is put ourselves at risk.”

It’s rare but not unprecedented for officers to face charges in situations like this. A school resource officer in Parkland, Florida is awaiting trial on charges of child neglect under the state’s “caregiver” law. He is accused of failing to act during the mass killing that left 17 people dead four years ago.