President Joe Biden tried to comfort families in the south Texas town of Uvalde on Sunday (May 29) after the nation's deadliest school shooting in a decade as federal officials announced they would review local law enforcement's slow response to the attack.

Anger has mounted over the decision by law enforcement agencies in Uvalde to allow the shooter to remain in a classroom for nearly an hour while officers waited in the hallway and children inside the room made panicked 911 calls for help.

The president and first lady Jill Biden wiped away tears as they visited memorials at the Robb Elementary School where the gunman killed 19 students and two teachers, laying white roses and paying respects to makeshift shrines to the victims.

“Do something,” a crowd chanted outside Sacred Heart Catholic Church as Biden exited after attending mass.

“We will,” he answered.

The Bidens met privately with victims' families and survivors for several hours before later meeting behind closed doors with first responders.

"We grieve with you. We pray with you. We stand with you. And we’re committed to turning this pain into action," Biden tweeted in the early evening before concluding his visit.

Police say the gunman, 18-year-old Salvador Ramos, entered the school on Tuesday with an AR-15 semi-automatic rifle after earlier shooting his grandmother, who survived.

Official accounts of how police responded to the shooting have flip-flopped wildly. The US Department of Justice on Sunday said it would review local law enforcement response at the request of Uvalde Mayor Don McLaughlin.