Joe Biden visited bereaved families after the worst school attack in a decade and vowed to “turn pain into action.”
The president and first lady visited a memorial at a Uvalde school before attending a church service and meeting with survivors and first responders.
The US Justice Department said it would review the Texas shooting response.
Mrs. Biden laid a bouquet of 21 white flowers, one for each child killed, as they paid their respects.
100 people chanted “do something” as they left the church.
“Well,” he said.
Later, Trump tweeted that he mourns, prays, and stands with Uvalde.
“We’re turning pain into action,” he said.
Investigators want to know how critical mistakes were made in the Robb Elementary School shooting response.
At the request of Uvalde’s mayor, the Justice Department will review law enforcement’s response to the shooting.
“The review will provide an independent account of law enforcement actions and responses that day and identify lessons learned and best practices to help first responders prepare for and respond to active shooter events,” he said.
“After its review, the Justice Department will publish a report”
The review will be fair, impartial, and independent, he said, and the results will be public.
Some have asked the FBI to investigate why the police allowed the shooter, Salvador Ramos, to remain in a classroom for nearly an hour while officers waited in the hallway and children made panicked 911 calls.
Police say the 18-year-old entered the school with an AR-15 after killing his grandmother at their home.
Official accounts of how police responded to the shooting have flip-flopped, prompting calls for an independent investigation.
Mr. Biden, a Democrat, has called for major gun law changes but has met Republican opposition.
After the shooting, Biden spoke at Delaware’s commencement.
“Evil came to that Texas classroom, that New York grocery store, and many other places where innocents died,” he said.
“We must be resolute.” Stronger. We can make America safer, but we can’t outlaw tragedy.
Mr. Biden’s visit to Texas is his third mass shooting site as president.
He visited Buffalo this month after a supermarket shooting that killed 10 black people.
In the wake of two back-to-back mass shootings, Vice President Kamala Harris called for a ban on assault-style weapons, saying they have “no place in a civil society”.