Panicked children and teachers placed half a dozen calls to 911 emergency services from the Texas classrooms where a massacre was unfolding, pleading for police to intervene, while roughly 20 officers waited in a hallway for nearly an hour before entering the room, authorities said on Friday.

At least two children called the 911 emergency number from the two connecting classrooms after 18-year-old Salvador Ramos entered with an AR-15 semi-automatic rifle, according to Colonel Steven McCraw, director of the Texas Department of Public Safety.

The on-site commander, the chief of the school district's police department in Uvalde, Texas, believed Ramos was barricaded inside the classroom and that children were no longer at risk, giving police time to prepare, McCraw said.

"From the benefit of hindsight where I'm sitting now, of course, it was not the right decision," McCraw said. "It was the wrong decision."

Some of the students trapped in the classrooms with the gunman survived the massacre, including at least two who called 911, McCraw said, though he did not offer a specific tally.

Someone whom McCraw did not identify called the 911 line multiple times starting at 12:03 p.m., telling police in a whisper that there were multiple dead and that there were still "eight to nine" students alive, the colonel said. One student called at 12:47 p.m. and asked the operator to "please send the police now."

Officers did not go into the classroom until 12:50 p.m., according to McCraw, when the U.S. Border Patrol tactical team used keys from a janitor to open the locked door and kill Ramos.

Several officers had an initial exchange of gunfire with Ramos shortly after he entered the school at 11:33 a.m., when two officers were grazed by bullets. There were as many as 19 officers in the hallway by 12:03 p.m., McCraw said - the same time that the first 911 call from inside the classroom was received.

Videos that emerged on Thursday showed frantic parents urging police to storm the school during the attack, with some having to be restrained by police.

Standard security protocols advise police to confront an active school shooter without delay, a point McCraw conceded on Friday.

"When there's an active shooter, the rules change," he said.

Police recovered 142 spent rounds inside the school from Ramos' rifle, as well as nearly two dozen more on school property outside the building, McCraw said.

In total, Ramos had 60 magazines and 1,657 rounds, including some left in his truck when he crashed it outside the school before the attack and two magazines recovered at his residence.