The shooter that killed 19 children at an elementary school in the US state of Texas posted on social media that he would shoot the school, the state's governor said Wednesday.

"There was no meaningful forewarning of this crime, other than what I'm about to tell you," Governor Greg Abbott said at a news conference. "As of this time, the only information that was known in advance was posted by the gunman on Facebook, approximately 30 minutes before reaching the school."

Abbott said the shooter said he was going to shoot his grandmother in an initial post and in a subsequent one said: "I shot my grandmother."

"The third post, maybe less than 15 minutes before arriving at school was 'I'm going to shoot an elementary school,’" said Abbott.

The shooting at Robb Elementary School in Uvalde, about 83 miles (133 kilometers) west of San Antonio, also left two adults dead. The shooter, 18-year-old Salvador Ramos was killed at the scene by a Border Patrol officer.

The governor said Ramos had no known mental health history and used an AR-15 in the killing spree./aa