Frantic children called 911 at least half a dozen times from the Texas classrooms where a massacre was unfolding, pleading for police to intervene, as some 20 officers waited in the hallway for nearly an hour before entering and killing the gunman, authorities said on Friday.

At least two children placed several emergency calls from a pair of adjoining fourth-grade classrooms after 18-year-old Salvador Ramos entered on Tuesday with an AR-15 semi-automatic rifle, Steven McCraw, the head of the Texas Department of Public Safety, said at a contentious news conference. 

The commander at the scene in Uvalde, the school district's police chief, believed that Ramos was barricaded inside adjoining classrooms at Robb Elementary School and that children were no longer at risk, McCraw said.

"It was the wrong decision. Period. There's no excuse for that," he added.

Friday's briefing came after authorities spent three days providing often conflicting and incomplete information about the more than an hour that elapsed between the time Ramos entered the school and when U.S. Border Patrol agents unlocked the classroom door and killed him.

Three police officers followed Ramos into the building within two minutes. In the next half hour, as many as 19 officers piled into the hallway outside. But another 47 minutes passed before the Border Patrol tactical team breached the door, McCraw said.

As the gunman fired at students, law enforcement officers from other agencies urged the school police chief to let them move in because children were in danger, two law enforcement officials said.

Ramos killed 19 children and two teachers inside the room. His motive remained unclear, authorities said.

There was a barrage of gunfire shortly after Ramos entered the classroom where officers eventually killed him, but those shots were "sporadic" for much of the time that officers waited in the hallway, McCraw said. He said investigators do not know if children died during that time.

Throughout the attack, teachers and children repeatedly called 911 asking for help.

Questions have mounted over the amount of time it took officers to enter the school to confront the gunman. During the siege, frustrated onlookers urged police officers to charge into the school, but the police's response was, "We can't do our jobs because you guys are interfering," according to witnesses.

(With input from agencies)