(NewsNation) — Responding officers in Uvalde, Texas, believed a gunman inside Robb Elementary School had barricaded himself inside a classroom and they mistakenly changed their approach to the active shooter incident.

“When there’s an active shooter, the rules change,” said Texas Department of Public Safety Director Steven McCraw. “It’s no longer a barricaded subject. You don’t have time. You don’t worry about perimeters.”

In an active shooter situation, NewsNation learned, law enforcement should try to eliminate the shooter as soon as possible. But the approach is much different in a barricaded subject or hostage situation.

Instead, officers are advised not to rush into a room while a hostage or barricade scenario is taking place, according to Pete Blair, the executive director of the Advanced Law Enforcement Rapid Response Training Center (ALERRT) at Texas State University.

Blair spoke with NewsNation Thursday about general protocol but did not draw any conclusions about the Uvalde shooting specifically. His interview was conducted before investigators released more detailed information Friday about their response at Robb Elementary.

“They could create a situation where the attacker starts shooting hostages,” Blair said. “So if the attacker is not killing people, the default is to kind of try to negotiate at the door and stall for time to get the SWAT team there to come in and deal with the situation.”

In Uvalde, the on-scene commander at the time of the shooting believed the situation had transitioned from an active shooter to a barricaded subject, according to McCraw. McCraw added that in hindsight, it was the wrong decision.

About 45 minutes passed before officers entered the room using keys from the janitor, police said. Shots during that time were sporadic and it’s unclear how many children died within that window, according DPS.

Again speaking about protocol and not Uvalde, Blair said in those types of situations, officers are trained to make an assessment.

“If they’re hearing gunfire, which means people might be getting killed, or if you know that there are injured people there in the room and every second you delay in time to get somebody to definitive care is a greater chance that person is going to die, then you can as a patrol officer push that situation,” Blair said.

A January 2020 Texas Commission on Law Enforcement training document gives similar guidance, noting that the number of deaths in an active shooter situation largely depends on two factors: how quickly the police engage them and how quickly the shooter can find victims.

Nineteen children and two teachers were killed in the Uvalde, Texas, shooting. Several more were injured, including the gunman’s grandmother, whom officers have said he shot in the face.

“Any time that something tragic like this happens, we want to know why it happened and if we can do better next time and call it like it is,” McCraw said. “It’s tragic.”