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UVALDE, TEXAS -- An 18-year-old gunman who killed 19 children and two teachers at a Texas elementary school barricaded himself inside a classroom, "shooting anyone that was in his way," an official said Wednesday, describing the latest in a gruesome, yearslong series of mass killings at churches, schools and stores.

Police and others responding to Tuesday's attack broke windows at the school in an effort to allow students and teachers inside to escape, Lt. Christopher Olivarez of the Texas Department of Public Safety said Wednesday on NBC's "Today."

Olivarez told CNN that all victims were in the same fourth-grade classroom at Robb Elementary School. Eventually, the shooter was killed by law enforcement.

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Tuesday's assault in the heavily Latino town of Uvalde was the deadliest shooting at a U.S. school since a gunman killed 20 children and six adults at Sandy Hook Elementary in Newtown, Connecticut, in December 2012.

Families waited hours for word on their children. At the town civic centre where some gathered, the silence was broken repeatedly by screams and wailing. "No! Please, no!" one man yelled as he embraced another man.

"My heart is broken today," said Hal Harrell, the school district superintendent. "We're a small community, and we're going to need your prayers to get through this."

Gov. Greg Abbott said one of the two adults killed was a teacher.

Adolfo Cruz, a 69-year-old air conditioning repairman, was still outside the school as the sun set, seeking word on his 10-year-old great-granddaughter, Eliajha Cruz Torres.

He drove to the scene after receiving a terrifying call from his daughter shortly following the first reports of the shooting. He said other relatives were at the hospital and the civic centre.

Waiting, he said, was the heaviest moment of his life.

"I hope she is alive," Cruz said.

The attack was the latest grim moment for a country scarred by a string of massacres, coming just 10 days after a deadly, racist rampage at a Buffalo, New York, supermarket. And the prospects for any reform of the nation's gun regulations seemed as dim, if not dimmer, than in the aftermath of the Sandy Hook deaths.

But U.S. President Joe Biden appeared ready for a fight, calling for new gun restrictions in an address to the nation hours after the attack.

"As a nation we have to ask, when in God's name are we going to stand up to the gun lobby? When in God's name are we going to do what has to be done?" Biden asked. "Why are we willing to live with this carnage?"

It was not immediately clear how many people in all were wounded, but the school district's police chief, Pete Arredondo, said there were "several injuries."

The attack on the school began about 11:30 a.m. The gunman had already shot his grandmother, according to Olivarez of the Department of Public Safety. After fleeing that scene, he crashed his car at the school and made his way inside.

Olivarez said that when local and state officers responded they heard gunshots -- and were shot at themselves.

"The shooter was able to make entry into a classroom, barricaded himself inside that classroom and again just began shooting numerous children and teachers that were in that classroom having no regard for human life. Just began shooting anyone that was in his way," he said. "At that point we had a tactical law enforcement arrive [and they] were able to make forcible entry into that classroom. They were met with gunfire as well but they were able to shoot and kill that suspect."

Earlier, a law enforcement official had said one Border Patrol agent who was working nearby when the shooting began rushed into the school without waiting for backup and shot and killed the gunman. The official spoke on condition of anonymity because he was not authorized to talk about it.

The agent was wounded but able to walk out of the school, the law enforcement official said.

Jason Owens, a top regional official with the Border Patrol, said some area agents have children at Robb Elementary.

"It hit home for everybody," he said.

Staff members in scrubs and devastated victims' relatives could be seen weeping as they walked out of Uvalde Memorial Hospital, which said 13 children were taken there. Another hospital reported a 66-year-old woman was in critical condition.

Officials did not immediately reveal a motive, but they identified the assailant as Salvador Ramos, a resident of the community about 85 miles (135 kilometres) west of San Antonio. Law enforcement officials said he acted alone.

Uvalde, home to about 16,000 people, is about 75 miles (120 kilometres) from the border with Mexico. Robb Elementary, which has nearly 600 students in second, third and fourth grades, is in a mostly residential neighborhood of modest homes.

The attack came as the school was counting down to the last days of the school year with a series of themed days. Tuesday was to be "Footloose and Fancy," with students wearing nice outfits.

Ramos had hinted on social media that an attack could be coming, according to state Sen. Roland Gutierrez, who said he had been briefed by state police. He noted that the gunman "suggested the kids should watch out," and that he had bought two "assault weapons" after turning 18.

Before heading to the school, Ramos shot his grandmother, Gutierrez said.

Other officials said that the grandmother survived and was being treated, though her condition was not known.

Investigators believe Ramos posted photos on Instagram of two guns he used in the shooting, and they were examining whether he made statements online in the hours before the assault, a law enforcement official said.

Law enforcement officers were serving multiple search warrants Tuesday night and gathering telephone and other records, the official said. Investigators were also attempting to contact Ramos' relatives and were tracing the firearms.

The official could not discuss details of the investigation publicly and spoke to The Associated Press on condition of anonymity.

Condolences poured in from leaders around the world. Pope Francis pleaded that it was time say "'enough' to the indiscriminate trade of weapons!" Foreign Minister Dmytro Kuleba of Ukraine, which is at war with Russia after Moscow invaded, said that his nation also knows "the pain of losing innocent young lives."

The tragedy in Uvalde was the deadliest school shooting in Texas history, and it added to a grim tally in the state, which has been the site of some of the deadliest shootings in the U.S. over the past five years.

In 2018, a gunman fatally shot 10 people at Santa Fe High School in the Houston area. A year before that, a gunman at a Texas church killed more than two dozen people during a Sunday service in the small town of Sutherland Springs. In 2019, another gunman at a Walmart in El Paso killed 23 people in a racist attack targeting Hispanics.

The shooting came days before the National Rifle Association annual convention was set to begin in Houston. Abbott and both of Texas' U.S. senators were among elected Republican officials who were the scheduled speakers at a Friday leadership forum sponsored by the NRA's lobbying arm.

GUN CONTROL DEBATE

In the years since Sandy Hook, the gun control debate in Congress has waxed and waned. Efforts by lawmakers to change U.S. gun policies in any significant way have consistently faced roadblocks from Republicans and the influence of outside groups such as the NRA.

A year after Sandy Hook, Sens. Joe Manchin, a West Virginia Democrat, and Patrick J. Toomey, a Pennsylvania Republican, negotiated a bipartisan proposal to expand the nation's background check system. But the measure failed in a Senate vote, without enough support to clear a 60-vote filibuster hurdle.

Last year, the House passed two bills to expand background checks on firearms purchases. One bill would have closed a loophole for private and online sales. The other would have extended the background check review period. Both languished in the 50-50 Senate, where Democrats need at least 10 Republican votes to overcome objections from a filibuster.

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Eugene Garcia and Dario Lopez-Mills in Uvalde, Jake Bleiberg in Dallas, Ben Fox, Michael Balsamo and Eric Tucker in Washington, Paul J. Weber in Austin, Juan Lozano in Houston, Gene Johnson in Seattle and Rhonda Shafner in New York contributed to this report.