UVALDE, Texas (AP) — The gunman who massacred 19 children and two teachers at an elementary school in Texas had warned in online messages minutes before the attack that he had shot his grandmother and was going to shoot up a school, the governor said Wednesday.

Salvador Ramos, 18, used an AR-15-style semi-automatic rifle in the bloodshed Tuesday at Robb Elementary School in Uvalde that ended with police storming a classroom and killing him. He had legally bought two such rifles just days before, soon after his birthday, authorities said.

“Evil swept across Uvalde yesterday. Anyone who shoots his grandmother in the face has to have evil in his heart,” Governor Greg Abbott said. “But it is far more evil for someone to gun down little kids.”

Investigators shed no light on the motive for the attack, which also left at least 17 people wounded. The governor said Ramos, a resident of the small town about 85 miles (135 kilometres) west of San Antonio, had no known criminal or mental health history.

But about a half-hour before the bloodbath, Ramos sent three messages online, Abbott said. Ramos wrote in the first that he was going to shoot his grandmother, then that he had shot the woman, and finally that he was going to shoot up an elementary school, according to Abbott. It was not clear whether Ramos specified which school.

Ramos sent private, one-to-one text messages on Facebook that were “discovered after the terrible tragedy,” company spokesman Andy Stone said. He said Facebook is cooperating with investigators.

Amid calls around the US for tighter restrictions on firearms, the Republican governor repeatedly talked about mental health struggles among Texas young people and argued that tougher gun laws in Chicago, New York and California are ineffective.

Democrat Beto O’Rourke, who is running against Abbott for governor, interrupted Abbott’s news conference and called the tragedy “predictable”. Pointing his finger at Abbott, he said: “This is on you until you choose to do something different. This will continue to happen.” O’Rourke was escorted out as members of the crowd yelled at him, with one man calling him a “sick son of a b*tch”.

Texas has some of the most gun-friendly laws in the nation and has been the site of some of the deadliest shootings in the US over the past five years.

As details of the latest mass killing to rock the US emerged, grief engulfed Uvalde, population 16,000.

The dead included an outgoing 10-year-old, Eliahna Garcia, who loved to sing, dance and play basketball; a fellow fourth-grader, Xavier Javier Lopez, who had been eagerly awaiting a summer of swimming; and a teacher, Eva Mireles, with 17 years’ experience whose husband is an officer with the school district’s police department.

“I just don’t know how people can sell that type of a gun to a kid 18 years old,” Eliahna’s aunt, Siria Arizmendi, said angrily through tears. “What is he going to use it for but for that purpose?”

Lt Christopher Olivarez of the Texas Department of Public Safety told CNN that all of those killed were in the same fourth-grade classroom.

The killer “barricaded himself by locking the door and just started shooting children and teachers that were inside that classroom,” Olivarez said. “It just shows you the complete evil of the shooter.”

Police and others responding to the attack also went around breaking windows at the school to enable students and teachers to escape. Law enforcement officers eventually broke into the classroom and killed Ramos in one last exchange of gunfire, authorities said.

The attack in the predominantly Latino town was the deadliest school shooting in the US since a gunman killed 20 children and six adults at Sandy Hook Elementary in Newtown, Connecticut, in December 2012.

The bloodshed was the latest in a seemingly unending string of mass killings at churches, schools, stores and other sites in the US. Just 10 days earlier, 10 black people were shot to death in a racist rampage at a Buffalo, New York, supermarket.