Authorities in Uvalde, Texas, have revealed more details about the mass shooting at an elementary school that claimed the lives of 19 students and two teachers.

They say some of the young victims called 9-1-1 during the attack to beg for help.

They're also raising more questions over the police response to the attack, in which the 18-year-old shooter entered the school freely and opened fire with an assault rifle.

A law enforcement official says 19 officers stood in a hallway for nearly for an hour before a tactical team broke into the classroom and killed the shooter on the spot.

The Director of the Texas Department of Public Safety, Steven McCraw, said the decision to wait was wrong and inexcusable.

McCraw said based on his information, he believes the officers should have entered the room as soon as possible.

He said the officers thought the gunman had barricaded himself in the room, and that the students were not at risk.

The attack has reignited the heated debate over gun ownership in the United States.

Lawmakers in Texas passed a measure last year that loosened restrictions on guns.

At a news conference, Governor Greg Abbot rejected the idea that it had anything to do with the shooting. Instead he pinned the blame on mental health issues.

But investigators say the gunman had no history of mental illness or criminal record. They're still trying to establish a motive.