Authorities say the victim death toll from the elementary school shooting now stands at 19 children and two adults.
The latest figures come from Travis Considine, a spokesman for the Texas Department of Public Safety. The gunman also died.
Authorities offered no names or descriptions of the two adults.
It 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.
And it came just 10 days after a gunman in body armour killed 10 black shoppers and workers at a supermarket in Buffalo, New York, in what authorities say was a racist attack.
“My heart is broken today,” said Hal Harrell, the school district superintendent, announcing that all school activities were cancelled until further notice.
“We’re a small community and we’re going to need you prayers to get through this.”
The gunman, who was wearing body armour and had hinted on social media of an upcoming attack, crashed his car outside the school and went inside armed, 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, Mr Gutierrez said.
“That was the first thing he did on his 18th birthday,” he said.
Federal law enforcement officials said the death toll was expected to rise. They spoke on condition of anonymity because they were not authorised to release investigative details.
The gunman entered Robb Elementary School in Uvalde with a handgun and possibly a rifle, Governor Greg Abbott 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 heavily Latino community about 85 miles (135 kilometres) west of San Antonio.
A Border Patrol agent who was 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 speaking on condition of anonymity because he was not authorised to talk about it.
The agent was wounded but able to walk out of the school, the law enforcement source said.
Mr Abbott said the gunman was likely killed by police officers but that the events were still being investigated. The school district’s police chief, Pete Arredondo, said that the attacker acted alone.
Hollywood stars including Taylor Swift, Simu Liu, and Amy Schumer have also expressed their shock and anger at the attack, as well as actor Matthew McConaughey, who is from the town of Uvalde, where the shooting took place.
James Corden said the thought of his child being in a school shooting is “beyond comprehension”.
The comedian and host of The Late Late Show said that on the issue of gun laws America is “one of the most backward places in the world”.
Speaking during a monologue after the recording of his show on Tuesday, Corden addressed the atrocity in Texas, which has reportedly killed 19 children.
“As a father, I can’t imagine the horror of that phone call,” he said.
“When I dropped my kids off at school this morning and kissed them goodbye, it doesn’t cross your mind that that could ever be the last goodbye.
“And the thought of that phone call, that your child is the victim in a mass shooting is beyond comprehension as a human being.”
He continued: “I’m so deeply sad for the families of these children, the trauma of the survivors and for the future these kids will never see.
“It doesn’t make sense to me. It doesn’t reflect the country that I think America is, the America I have always admired.
“You have a problem, you solve it.
“Yet on this issue, America is one of the most backward places in the world.
“This year, there have been no school shootings in England, there have been no school shootings in Japan. This year, there have been no school shootings in Australia.
“This year, there have been 27 school shootings in America and 212 mass shootings and we are just five months into the year.”
He added: “I’ll probably be stood here in a week or two, talking about another place.
“Words of thoughts and prayers will come from our leaders but I fear change never will.
“I hold out hope that this country will eventually change this senseless gun culture and my heart simply goes out to every person in Texas tonight.”