Nineteen students and two teachers were killed at police waited outside the school (Pictures: AP/Reuters)

Nineteen police officers stood outside the Texas elementary school as a gunman killed 19 children and two teachers, and waited an hour to break through a lockd door after an incident commander believed no more lives were at risk.

Frantic 911 calls from children locked inside the classroom with the gunman, however, said otherwise, officials in Texas admitted on Friday.

‘With the benefit of hindsight, where I’m sitting now, of course it was not the right decision. It was the wrong decision,’ said Steve McCraw, director of the Texas Department of Public Safety.

‘Clearly there were kids in the room. Clearly they were at risk,’ he said.

At least two children called 911 from inside the fourth grade classroom at Robb Elementary School in Uvalde, Texas, McCraw said. Both of the students who are known to have called for help survived.

One of those students, Miah Cerrillo, repeatedly pleaded with the dispatcher, ‘please send help because we’re in trouble.’ She covered herself in a dead friend’s blood in order to appear dead to the gunman.

Numerous startling details were revealed by McCraw during Friday’s briefing.

The incident commander, who he did not name, had determined that the situation had shifted from an active shooter to a barricaded subject, McCraw said.

Because of this, the commander believed there was time to retrieve keys to the classroom door from a janitor, and for the Border Patrol tactical team to arrive and enter the classroom behind shields.

This is a breaking news story, check back for updates…