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The US Department of Justice says it will investigate the police response to last week's 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.
In Uvalde, President Joe Biden met the victims' families on Sunday.
He also met 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 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 Department of Justice 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 on 24 May 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.
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".
Several senior Republicans have already pushed back against calls for tighter rules on gun ownership, such as background checks.
On Friday, former US President Donald Trump told the National Rifle Association's annual conference that decent Americans should be allowed firearms to defend themselves against "evil".
And Texas Senator Ted Cruz has accused Democrats and the media of seeking to "politicise" the shooting in Uvalde to "restrict the constitutional rights of law abiding citizens".
Speaking to the BBC at a memorial site in Uvalde ahead of Mr Biden's visit, several mourners expressed support for stricter gun laws.
Melissa Rangel said she was in favour of age checks, questioning "how at 18 years old you're able to buy a gun".
San Antonio resident Eduardo Messa said: "I mean, we have age limits for things like cigarettes and alcohol, why not for guns, you know, this is just... this kind of massacre that occurred this week is just so heart-breaking and sad."
However, Tad Neutze, a local businessman, told the BBC that he was a gun owner and challenged the need for increased checks. "If the guy was evil, he was an evil person. And evil is going to find a way to harm people," he said.
On Saturday, Vice-President Kamala Harris made an impassioned plea for a ban on assault weapons while attending the funeral of Ruth Whitfield, an 86-year-old killed in a mass shooting in Buffalo, New York, on 14 May. The attack at a supermarket in a predominantly black neighbourhood is believed to have been racially motivated. The teenaged suspect had also legally bought an AR-15-style weapon.
"Do you know what an assault weapon is?" Ms Harris asked mourners. "It was designed for a specific purpose: to kill a lot of human beings quickly. An assault weapon is a weapon of war, with no place, no place in a civil society."
Ms Harris also called for enhanced background checks on all firearms purchases.
But for Patricia Castanon, whose niece Annabelle Rodriguez died at Robb Elementary School, there is little President Biden can do to ease her anguish.
"He can't bring her back. He can't bring none of them back and nobody can," she told the BBC earlier on Sunday.