(NewsNation) — As many as 19 police officers waited for more than 45 minutes in a hallway outside the classrooms where an active shooter killed 19 children and two teachers, despite numerous 911 calls from students inside, a top public safety official told reporters Friday.

Within minutes of arriving at Robb Elementary School, the suspected 18-year-old shooter fired more than 100 rounds, said Texas Department of Public Safety Director Steve McCraw.

Police initially pursued the gunman but fell back after the suspect fired at them and injured two officers.

Once the shooter moved inside the adjoining classrooms, locking the doors to Rooms 111 and 112, McCraw said the shooting became more “sporadic,” leading officers to believe they were dealing with a barricaded subject rather than an active shooter.

For that reason, the incident commander, whom McCraw did not name, chose not to breach the classroom until additional tactical units arrived.

During that time, numerous 911 calls made by students in the classrooms suggest police knew there were at least eight to nine students alive. In one call, a student pleaded with dispatch to “please send the police.”

From the time the first 911 call inside the classroom was received, it took more than 45 minutes for law enforcement to get inside. Eventually they did so with a janitor’s master key.

“With the benefit of hindsight, where I’m sitting now, of course it was not the right decision,” McCraw said. “It was the wrong decision, period. There’s no excuse for that.”

It’s unclear how many students, if any, were killed between the first 911 call from inside the classroom at 12:03 p.m. and the time police entered the room at 12:50 p.m.

Both children who called 911 survived the massacre, according to McCraw.

This what we know about those calls.

12:03 p.m. Caller No. 1 — student inside Room 112

At 12:03 p.m. a student calls 911 and says she is in Room 112. Approximately seven minutes later, she calls back and tells dispatch that there are multiple people dead in the classroom.

On her fourth call, at 12:16 p.m., the student tells dispatch that there are eight to nine students alive in the classroom.

12:19 p.m. Caller No. 2 — student inside Room 111

At 12:19 p.m., a second student inside Room 111 calls 911 but hangs up after a student tells her to. Two minutes later, the same student calls back and three gunshots are overheard.

12:36 p.m. Caller No. 1 — student inside Room 112

At 12:36 p.m. the first caller tells 911 that “he shot the door” and is told to remain on the line and be quiet. Seven minutes later, the student asks dispatch to “please send the police now.” About five minutes later, she asks for police again.

At 12:50 p.m. shots are overheard on the phone. Shortly after, dispatch hears the sound of police moving children out of the classroom.