Washington DC - The US Justice Department will launch an investigation into if and how local police failed to stop or slow the massacre at Robb Elementary School in Texas last week.

"At the request of Uvalde Mayor Don McLaughlin, the US Department of Justice will conduct a Critical Incident Review of the law enforcement response to the mass shooting in Uvalde, Texas," officials said Sunday.

Nineteen fourth-grade students and two teachers were fatally shot Tuesday inside their classroom as police waited outside.

"The goal of the review is to provide an independent account of law enforcement actions and responses that day, and to identify lessons learned and best practices to help first responders prepare for and respond to active shooter events," the Department of Justice said.

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A report will be released at the end of the review.

After days of changing stories, Steven McCraw, the Texas Department of Public Safety director, said Friday that the commander on the scene of the primary school thought "there were no more children at risk," meaning they had time to wait for a key to get into the classroom where 18-year-old Salvador Ramos had barricaded himself and his victims.

"With the benefit of hindsight ... of course it was not the right decision. It was the wrong decision," McCraw said.

At the same time the commander decided to wait, at least two children trapped inside were calling 911 and begging for help, according to officials.