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.
US President Joe Biden is in Uvalde to meet families devastated by the attack.
He will also meet survivors and first responders.
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.
Both were seen wiping tears from beneath their sunglasses. Mrs Biden touched each child’s photo in turn.
The couple then attended a Catholic Mass at the local Sacred Heart church. Protesters outside the church shouted, “Do something!” as the president walked out.
“We will,” he answered.
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 shot his grandmother, and then 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.
BBC /Taiwo Akinola
