An 18-year-old gunman opened fire Tuesday at a Texas elementary school, killing at least 19 children as he went from classroom to classroom, officials said, in the latest gruesome moment for a country scarred by a string of massacres. The attacker was killed by law enforcement.

The death toll also included two adults, authorities said. Governor Greg Abbott said one of the two was a teacher.

The assault at Robb Elementary School in the town of Uvalde was the deadliest shooting at a US grade school since a gunman killed 20 children and six adults at Sandy Hook Elementary in Newtown, Connecticut, almost a decade ago.

“My heart is broken today,” said Hal Harrell, the school district superintendent, announcing that all school activities were canceled until further notice. “We’re a small community, and we’re going to need your prayers to get through this.”

The attack came just 10 days after a deadly, racist rampage at a Buffalo, New York, supermarket that added to a yearslong series of mass killings at churches, schools and stores. However, the prospects for any reform of US gun regulations seem as dim as in the aftermath of the Sandy Hook massacre.

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?” the US leader asked. “Why are we willing to live with this carnage?"

Many of the injured were rushed to Uvalde Memorial Hospital, where staff in scrubs and devastated victims' relatives could be seen weeping as they walked out of the complex.

The gunman, who was wearing body armor, crashed his car outside the school before going inside, Sergeant Erick Estrada of the Texas Department of Public Safety told CNN.

He killed his grandmother before heading to the school with two military-style rifles he had purchased on his birthday, according to state Senator Roland Gutierrez, who said he had been briefed by state police.

“That was the first thing he did on his 18th birthday,” he said.

Officials did not immediately reveal a motive, but the governor identified the assailant as Salvador Ramos and said he was a resident of the community about 135 kilometers west of San Antonio.

Ramos had hinted on social media that an attack could be coming, Gutierrez said, noting that “he suggested the kids should watch out.”

A 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, who was behind a barricade, according to a law enforcement official.

The agent was wounded but able to walk out of the school, the source said.

The school district's police chief, Pete Arredondo, said that the attacker acted alone.

It was not immediately clear how many people were wounded, but Arredondo said there were “several injuries.” Earlier, Uvalde Memorial Hospital said 13 children were taken there. Another hospital reported a 66-year-old woman was in critical condition.

Robb Elementary School has an enrollment of just under 600 students, and Arredondo said it serves students in the second, third and fourth grade. He did not provide ages of the children who were shot. This was the school's last week of classes before summer break.

White House press secretary Karine Jean-Pierre said Biden was briefed on the shooting on Air Force One as he returned from a five-day trip to Asia.

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 US over the past five years.

It came days before the National Rifle Association annual convention was set to begin in Houston. Abbott and both of Texas’ US senators are among elected Republican officials scheduled to speak at a Friday leadership forum sponsored by the NRA’s lobbying arm.

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 lobby groups such as the NRA. (AP)