Newly released police communication reveals they almost certainly knew children had been injured or killed, raising further questions about why they didn't storm the school immediately, as ITV News US Correspondent Robert Moore reports
It comes after police admitted it was "the wrong decision" for officers to wait outside a classroom for nearly an hour before they entered and killed the gunman.
Nearly 20 officers stood outside the classroom for 48 minutes before US border patrol agents used a master key to open the locked door, shooting dead 18-year-old gunman Salvador Ramos.
Texas Department of Public Safety Director Steven McCraw said on Friday that the commander had assumed it was no longer an active shooter situation and that Ramos was barricaded inside.
But he added "there were in fact children in the classroom," and said investigators did not know if or how many children died during those 48 minutes.
Throughout the shooting, teachers and children repeatedly called 911 asking for help, including a girl who pleaded, “please send the police now."
Biden visits the scene
President Joe Biden and first lady Jill Biden offered comfort to those gripped by grief as they paid their respects at a memorial to the victims at Robb Elementary School on Sunday.
The trip to Uvalde was the president's second trip in two weeks to the scene of a mass shooting.
He travelled to Buffalo, New York, on May 17, to meet with victims' families after a gunman who cited a far-right conspiracy theory killed 10 black people at a supermarket.
Mr Biden stopped at the memorial of 21 white crosses, while the first lady placed down a bouquet of white flowers.
The couple viewed individual altars erected in memory of each student, and the first lady touched the children's photos as they moved along the row.
The president is due to meet the families of those killed, as well as survivors and first responders.
Mckinzie Hinojosa, whose cousin Eliahana Torres was killed Tuesday, said she respected Biden’s decision to mourn with the people of Uvalde.
“It’s more than mourning,” she said. “We want change. We want action. It continues to be something that happens over and over and over. A mass shooting happens. It’s on the news. People cry. Then it’s gone. Nobody cares. And then it happens again. And again.”
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The USA's gun divide
The shootings in Texas and New York have only served to deepen America's entrenched divisions when it comes to gun violence.
“Evil came to that elementary school classroom in Texas, to that grocery store in New York, to far too many places where innocents have died,” Biden said on Saturday.
“We have to stand stronger. We must stand stronger. We cannot outlaw tragedy, I know, but we can make America safer.”
Ramos legally purchased two guns not long before the school attack: an AR-style rifle on May 17 and a second rifle on May 20, authorities said. He had just turned 18, permitting him to buy the weapons under federal law.
Hours after the shooting, Biden delivered an impassioned plea for additional gun control legislation, asking: “When in God’s name are we going to stand up to the gun lobby? Why are we willing to live with this carnage? Why do we keep letting this happen?”
But several senior Republicans have already pushed back against appeals for tighter gun control, including against background checks.
On Friday, former US president Donald Trump told the National Rifle Association's annual conference that "the only way to stop a bad guy with a gun is a good guy with a gun."
He also said law-abiding Americans should be allowed guns to defend themselves against "evil" and called for teachers to be armed.