Policing experts said if officers in Texas didn't engage the gunman, they were violating training.
Witnesses said parents begged officers to go into the elementary school while the gunman was inside.
If someone is shooting, "there is no 'let's pause and get backup,'" one expert told Insider.
Experts on police training told Insider that if officers in Uvalde, Texas, delayed storming Robb Elementary School after a gunman entered the building, they would not have been following standard protocol. Onlookers at the school told the Associated Press and other news outlets that police didn't enter the building for up to 45 minutes while the gunman was inside.
Alex del Carmen, the associate dean of the school of criminal justice at Tarleton State University in Stephenville, Texas, told Insider that the "universally accepted standard is that when an active shooter shows up, what will happen is law enforcement will engage the suspect until he is neutralized."
Del Carmen said police are usually trained to follow and engage a suspect until there is no longer a threat.
Kalfani Turè, a policing expert and assistant professor of criminology at Mount Saint Mary's University in Maryland, agreed, telling Insider: "You've got to pursue."
"That is the simple policy, no question," Turè said. "You have to enter the school and pursue the individual with high-power firearms to finality. This is a situation where you have to be a superhero."
Turè said that while police officers are trained to preserve their own lives so they can continue to be helpful in an active situation, their "job is not only to preserve your life but to also preserve the life of others."
The Uvalde Police Department did not respond to multiple requests for comment.
Del Carmen added that when a shooter barricades himself inside a building, law-enforcement officers are taught to evacuate and call the SWAT team to negotiate with a suspect and hopefully reach a peaceful conclusion in which the suspect is taken into custody and no one is hurt.
When the suspect is barricaded with hostages, "a tactical unit should respond, circle around the area, negotiate with the person, and hope a peaceful resolution will come about," del Carmen told Insider.
Steve McCraw, the director of the Texas Department of Public Safety, said that the gunman in Uvalde was barricaded inside a classroom with students and teachers for about 40 minutes before a US Border Patrol team entered the room and shot and killed him.
McCraw said that a Uvalde school district officer approached the gunman outside the school, and the New York Times reported that Texas law-enforcement officials told reporters at a Thursday press conference that two other officers exchanged fire with the suspect but were shot and had to fall back.
Parents and bystanders at the scene, however, have said police delayed entering the building despite pleas to do so.
"They said they rushed in and all that, we didn't see that," Javier Cazares, whose 9-year-old daughter was killed in the shooting, told the Times.
Ultimately, the gunman fatally shot 19 children and two teachers inside a classroom of the elementary school.
Police haven't explained why it took 40 minutes to breach the classroom.
"If you have someone armed and shooting at victims, there is no 'let's pause and get backup,'" del Carmen told Insider.
Del Carmen said that if a suspect presents a weapon, police officers are trained to shoot at the gunman's torso to neutralize their central nervous system, disarm them, and put them on the ground to mitigate the threat.
"You have every legal and industry-standard backing to stop the threat," del Carmen said. "Unless there was something on the scene they perceived that would cause more harm to engage the suspect than to wait for backup, you should engage."
"Our hope is that the training we render and give police officers becomes the norm and that in these situations, this becomes 'muscle memory,'" del Carmen said.
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