The US Justice Department will investigate the police response to the mass shooting at a school in Uvalde, Texas which killed 19 children and two teachers.

Public anger is growing after it emerged that officers waited in the hallway as children trapped with the shooter made desperate 911 calls.

First Lady Jill Biden, herself a teacher, accompanied the president to a memorial at Robb Elementary School. They were seen comforting the school’s principal Mandy Gutierrez, beside a carpet of floral tributes for the teachers, and children – all under 10 years old – who lost their lives.

The president is expected to attend a Catholic Mass in the community later on Sunday.

Announcing its Critical Incident Review on Sunday, the US Justice Department said the goal was 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 shooting has provoked new calls for gun control measures, in a country reeling from two vicious shootings in under 10 days – although leading Republicans oppose tightening rules.

The US has now surpassed 200 mass shootings since the beginning of 2022.

A mass shooting is defined as an incident in which four or more people are shot or killed, excluding the shooter.

White House officials say Mr Biden is unlikely to offer specific policy proposals or seek to issue an executive order in the coming weeks to avoid interfering with delicate negotiations between Senate Democrats and Republicans.

The president’s visit comes days after 18-year-old Salvador Ramos opened fire on a classroom of fourth-graders with a legally acquired AR-15 style assault rifle.

The gunman’s rampage lasted for over an hour and police found as many as 1,657 rounds of ammunition and 60 magazines in his possession after he was shot dead.

Authorities have struggled to give a clear timeline of how events unfolded in Uvalde.

On Friday officials admitted that police had delayed entering the school for over 40 minutes because they did not believe it was still an “active shooter” situation.

The senior officer on the scene decided to wait until the school janitor arrived with the keys because they thought that either “no kids were at risk” by then, or “no-one was living anymore”.