The US Justice Department will launch an independent assessment of the police response to the recent mass shooting at a Texas elementary school, amid rising doubts over police efforts during the tragedy.

Questions have mounted over the amount of time it took officers to enter the school to confront the gunman.

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Anthony Coley, Department of Justice spokesperson, stated that "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."

Uvalde Mayor Don McLaughlin requested the inquiry, which "will be fair, transparent and independent," according to Coley.

On Tuesday, a young shooter attacked Robb Elementary School in the small Texas town of Uvalde, murdering 19 students and two teachers, the latest in a nationwide spate of lethal mass shootings.

Following the incident, police actions have come under intense criticism as tales of their sluggish response surface.

Texas officials confirmed Friday that up to 19 cops remained in the school corridor for over an hour before breaking into where the gunman was and shooting him dead, claiming that the officers incorrectly assumed he had stopped shooting and was now trapped.

Officials are now calling the delay a "wrong decision", which has completely outraged parents.