The US Justice Department has said it will review law enforcement's response to the school shooting in Uvalde, Texas, that killed 19 children and two teachers, amid growing anger over why police failed to swiftly confront the gunman.
Uvalde law enforcement agencies allowed the shooter to remain in a classroom at Robb Elementary School for nearly an hour while officers waited in the hallway and children inside made panicked 911 calls for assistance.
Texas state and county authorities say they are conducting their own inquiry.
Anthony Coley, a spokesman for the US Justice Department, said it will conduct a "Critical Incident Review" of law enforcement's response to the shooting at the request of Uvalde's mayor.
"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," Mr Coley said.
"The Justice Department will publish a report with its findings at the conclusion of its review," Mr Coley added.
The review was announced after US President Joe Biden earlier laid flowers and prayed at the makeshift shrine erected in Uvalde to the 19 children and two teachers murdered by a teen gunman after he stormed their elementary school.
Mr Biden, accompanied by his wife, Jill Biden, was in the small Texas town less than two weeks after making a similar trip to the site of another mass shooting in Buffalo, New York.
Both wearing black, the first couple held hands in front of a memorial outside Robb Elementary School and walked slowly along the thicket of wreaths, bouquets, white crosses and blown-up photos of the slain children.
Mr Biden next met privately with first responders and grieving relatives of the dead.
Applause broke out from a crowd gathered at the school when the Bidens' motorcade arrived. However, illustrating the tension in the town after the massacre, there were boos at the appearance of Republican Texas Governor Greg Abbott.
"We need changes," shouted one man.
Mr Biden was not scheduled to speak publicly in Texas, but yesterday he renewed his call for Congress to overcome years of paralysis to toughen firearms regulations.
"We cannot outlaw tragedy, I know, but we can make America safer. We can finally do what we have to do to protect the lives of the people and of our children," Biden said in a speech at the University of Delaware.
Harrowing accounts emerged of the ordeal faced by survivors of Tuesday's attack, where the behavior of the police is under severe scrutiny.
Ten-year-old Samuel Salinas was sitting in his fourth-grade classroom when the shooter, later identified as Salvador Ramos, barged in and announced: "You're all going to die."
Then "he just started shooting," Salinas told ABC News.
Texas authorities admitted on Friday that as many as 19 police officers were in the school hallway for nearly an hour before finally breaching the room and killing Ramos, saying the officers mistakenly thought that he had stopped killing and was now barricaded.
Officials now call this delay a "wrong decision" but parents have expressed fury.
Survivors describe making desperate, whispered pleas for help in 911 phone calls during the lengthy assault, while some played dead to avoid drawing the shooter's attention.
The Uvalde shooting was the deadliest school attack since 20 children and six staff were killed at Sandy Hook Elementary School in Newtown, Connecticut in 2012.
But despite the epidemic of mass shootings and ever-growing flood of private gun purchases, Congress has repeatedly failed to agree on possible new regulations.
This time might be different, some lawmakers say.
Democratic Connecticut Senator Chris Murphy said today there were "serious negotiations" under way involving members of both parties.