Eighteen children and three adults were killed by a teenage gunman at an elementary school in South Texas on Tuesday, officials said, in the latest spasm of mass gun violence sweeping the United States.

The attacker was killed by law enforcement.

Governor Greg Abbott said the suspect, identified as 18-year-old Salvador Ramos, was apparently killed by police officers responding to the scene, and that two officers were struck by gunfire, though the governor 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, reports Reuters.

The shooting unfolded just 10 days after 10 people were killed in Buffalo, New York, in a predominantly Black neighborhood. An 18-year-old man whom authorities said opened fire with an assault-style rifle has been charged.

The motive for Tuesday's massacre in Texas, the latest in a string of seemingly random mass shootings that have become commonplace in the United States, was not immediately known.

Official details remained sketchy about the circumstances of the late morning shooting at Robb Elementary School in the town of Uvalde, Texas, about 80 miles west of San Antonio.

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.

Investigators believe Ramos shot and killed his grandmother before going to the school, CBS News reported, citing unidentified law enforcement sources.

"It is being reported that the subject shot his grandmother right before he went into the school," Abbott told reporters. "I have no further information about the connection between those two shootings."

University Hospital in San Antonio said on Twitter it had received two patients from the shooting in Uvalde, a 66-year-old woman and a 10-year-old girl, both listed in critical condition.

US President Joe Biden, who ordered flags flown at half-staff until sunset daily until May 28 in observance of the tragedy, planned to address the nation about the shooting at 8:15 pm EDT, the White House said.

The student body at the school consists of children in the second, third and fourth grades, according to Pete Arredondo, chief of the Uvalde Consolidated Independent School District Police Department, who also addressed reporters. In American schools, those grades are typically made up of children ranging from 7 to 10 years of age.