The U.S. leader and his wife plan to be in the small southwestern city for several hours, talking with those most affected by the carnage that ensued after an 18-year-old gunman burst into a fourth-grade classroom at Robb Elementary School and opened fire. Biden is expected to attend a Catholic Mass and meet with first responders.

It is the second time this month that Biden and the U.S. have been confronted with a mass killing. He earlier visited Buffalo, New York, in the northeastern United States, where a white supremacist opened fire, targeting and killing 10 Black people in a grocery store.

In the Texas shooting, law enforcement officials are being sharply questioned about why it took so long, more than an hour, to confront the gunman.

Even as children trapped in the classroom with the shooter made urgent emergency calls, pleading with police to rescue them, the incident commander on the scene, the police chief for Uvalde schools, assessed — wrongly, as it turned out — that it was no longer an active shooter incident but rather that the assailant, Salvador Ramos, had barricaded himself in the classroom.

As a result, the incident commander, Pete Arredondo, did not immediately order police officers into the classroom to confront the shooter.

Eventually, U.S. Border Patrol agents arrived at the school, burst in the classroom and killed Ramos, a high school dropout who bought two assault rifles earlier this month, a few days after he turned 18. Authorities say he had no criminal record and had not been under mental health treatment.

The head of the Texas Department of Public Safety, Steven McCraw, said Friday that with the benefit of hindsight, “it was the wrong decision” to wait to confront the shooter.

Lawmakers in Washington have long been stalemated over tightening gun purchase laws, with Democrats mostly supporting calls for stricter measures and background checks on gun buyers and Republicans almost universally opposed. In the aftermath of the Uvalde killings, a bipartisan group of Republican and Democratic senators is meeting to try to determine the scope of what new legislation could win congressional approval.

A longtime gun control proponent, Connecticut Democratic Senator Chris Murphy, told ABC’s “This Week” show Sunday, “We need federal legislation. There are more Republicans interested in talking this time” than after past mass shootings.

Raising the age limit for assault weapons purchases or banning their sale altogether—as was done in the U.S. from 1994 to 2004 — are unlikely to be part of any new legislation. Whether any of the possible changes could have prevented the Texas massacre is questionable since Ramos had no criminal record and had not been flagged for treatment.

Dozens of people gathered Saturday in Uvalde to mourn and pay homage to the 21 people killed last week.

Twenty-one crosses have been placed around a fountain in the city’s courthouse square, one for each of the 19 fourth graders who died and their two teachers, Irma Garcia and Eva Mireles. A growing pile of flowers, stuffed animals and messages — “Love you,” “You will be missed” — surrounded the crosses. Dozens of candles burned like small eternal flames.

Pastor Humberto Renovato, 33, who lives in Uvalde, asked everyone to join hands and pray.

The investigation continued Saturday into the time it took for police to confront the gunman.

Some 90 minutes elapsed between the beginning and the end of the deadly shooting. Ramos crashed a pickup into a ditch near the school, entered the building carrying an AR-15-style rifle and a bag of ammunition and was inside the school for 40 minutes to an hour before Border Patrol agents stormed in and killed him.

Samuel Salinas, 10, said Ramos barged into his fourth-grade classroom and said, “You’re all going to die.”

Then “he just started shooting,” Salinas told ABC News.

Another student, Daniel, whose mother allowed him to speak to The Washington Post, was in a classroom down the hall. He said his teacher, who quickly locked the door and turned out the lights, saved their lives. She was shot twice when the gunman fired through the door’s glass window, Daniel said.

For an hour, he said, the students hid in the dark. The only sounds in the room were hushed sobs and his teacher urging the students to remain quiet.

“’Stay calm. Stay where you are. Don’t move,'” Daniel recalled her saying.

Daniel told the newspaper that he and his classmates were rescued when police broke the room’s windows and they crawled to safety.

The city’s 911 call center received cries for help from at least two students in the adjoining classrooms where Ramos found his victims, Colonel Steven McCraw, director of the Texas Department of Public Safety, said earlier this week.

“He’s in Room 112,” one girl whispered to the 911 operator at 12:03 p.m. local time last Tuesday.

She called again at 12:43 p.m., begging the operator to “please send the police now,” and again four minutes later.

At 12:51 p.m., a Border Patrol-led tactical team stormed in and ended the siege.

Police have not yet found a motive for the shootings.

Ramos’s mother has asked the school children’s parents for forgiveness. In an interview with Televisa, a CNN affiliate, a soft-spoken Adriana Martinez said in Spanish, “I don’t know what he was thinking. … Forgive me. Forgive my son.”

With the Uvalde police under sharp criticism for their response to the shootings, police officers from other cities, including Houston and Dallas, have come to Uvalde to support and in some cases protect the officers of the police department, the mayor and the owner of the gun shop where the assailant bought his rifles and ammunition.

Arredondo has not commented publicly about his command of the police response to Ramos’s intrusion into the school. Uvalde officers have been stationed outside his home but would not say why.

New York City defense attorney Paul Martin and Chuck Wexler, executive director of the Police Executive Research Forum in Washington, both told The Associated Press Saturday that criminal charges are rarely filed against officers who fail to act in a mass shooting. It’s a “very high bar” because police officers are given wide latitude to make tactical decisions, Martin said, but they can be found civilly liable.

Biden said at a University of Delaware commencement Saturday that there has been “too much violence. Too much fear. Too much grief.”

Some information for this report came from The Associated Press and Reuters.