Governor Greg Abbott said the suspect, identified as 18-year-old Salvador Ramos, apparently was killed by officers called to Robb Elementary School on Tuesday in the town of in the town of Uvalde, Texas, about 128km west of San Antonio.
Two of those officers were struck by gunfire, though Abbott said their injuries were not serious. Authorities said the suspect acted alone.
Abbott told a news conference hours after the shooting that 14 schoolchildren had been slain, along with one teacher. But Texas state Senator Roland Gutierrez later told CNN, citing the Texas Rangers state police as his source, that the death toll had climbed to 18 children and three adults.
The school serves about 570 children in second through fourth grades, nearly 90% of them Hispanic. In American schools, those grades are typically made up of children ranging from 7 to 10 years of age.
Official details remained sketchy about the circumstances of the late-morning shooting. Abbott said the suspect was believed to have abandoned his vehicle and entered the school armed with a handgun, and possibly a rifle, before opening fire.
Texas Public Safety Department Sergeant Erick Estrada, appearing on CNN for an interview, said police saw the suspect emerge from a car he crashed near the school, carrying a rifle and a backpack, and "engaged" with him before he entered the south side of the school and started firing. Estrada said the gunman was wearing body armour.
Investigators believe Ramos shot and killed his grandmother before going to the school, CBS News reported, citing unidentified law enforcement sources.
The motive for the carnage was not immediately known.
"He shot and killed horrifically, incomprehensibly," Abbot told media. "Mr Ramos, the shooter, he himself is deceased and it's believed that responding officers killed him".
University Hospital in San Antonio said on Twitter it had received two patients from the shooting in Uvalde, one child and one adult. Both patients, a 66-year-old woman and a 10-year-old girl, were listed in critical condition.
The day's horrors were reflected on the school's Facebook page.
A few days ago, its posts showed the usual student activities - a trip to the zoo for second-graders and a save-the-date for a gifted-and-talented showcase. But on Tuesday, a note was posted at 11.43am: "Please know at this time Robb Elementary is under a Lockdown Status due to gunshots in the area. The students and staff are safe in the building," it read.
A second post was more explicit: "There is an active shooter at Robb Elementary. Law enforcement is on site." Administrators asked parents to stay away. And then finally, a note was posted advising parents that they could meet their children at the small city's civic centre.
The Texas rampage capped a series of mass shootings in US schools that have shocked the world and fuelled a fierce debate between advocates of tighter gun controls and those who oppose any legislation that could compromise the right of Americans to bear arms.
Tuesday's shooting is one of the deadliest at a US school since a gunman killed 26 people, including 20 children from 5- to 10-years old, in a rampage at Sandy Hook Elementary School in Connecticut in December 2012.
In 2018, a former student at Marjory Stoneman Douglas High School in Parkland, Florida, killed 17 students and educators.
Another crisis for Biden
US President Joe Biden, who ordered flags flown at half-staff until sunset daily until May 28 in observance of the tragedy, was to address the nation later on Tuesday, the White House said.
Biden spoke with Governor Abbott from Air Force One and offered him "any and all assistance" needed.
"Enough is enough," Vice President Kamala Harris said before Biden spoke. "As a nation, we have to have the courage to take action and understand the nexus between what makes for reasonable and sensible public policy to ensure something like this never happens again."
The latest shooting hands Biden, already facing the lowest approval ratings of his presidency, another crisis on top of 40-year high inflation rates and the war in Ukraine.
It came just 10 days after a mass shooting in a Buffalo, New York, grocery store, and piled pressure on his administration to make good its vows to crack down on gun violence.
When he ran for the presidency, Biden promised to push gun safety measures and reduce the country's tens of thousands of annual gun deaths. Biden and his fellow Democrats have failed to get enough votes in Congress for background checks for gun purchases or other proposed bills.
The United States is the most heavily armed society in the world, according to the Geneva-based Small Arms Survey, a research group.
Small, rural states where gun ownership is widespread have disproportionate influence in the Senate, where a supermajority of 60 votes is needed to advance most legislation in the 100-seat chamber.
The US experienced 61 "active shooter" incidents last year, up sharply from the prior year and the highest tally in over 20 years, the FBI reported this week.
