The gunman was spotted minutes later crashing his car into a ditch and running toward Robb Elementary School with a rifle. An officer with the Uvalde school district exchanged fire with him before he entered the school.

But at least 40 minutes passed – the shooter went through a back door, down two short hallways and ended up in a classroom where he opened fire – before a border patrol officer shot and killed him.

Texas Governor Greg Abbott said at a news conference that Ramos was a high school dropout with no criminal record or mental health history. He gave no warning of his crime, Abbott said, until about 30 minutes before he reached the school, when he posted on Facebook that he was going to shoot his grandmother, who had worked as a teacher’s aide for the junior school until 2020.

Biden demands US ‘stand up’ to gun makers after Texas massacre

In a second post on Facebook, the gunman said: “I’ve shot my grandmother”.

Less than 15 minutes before arriving at school, he said, Ramos posted: “I’m going to shoot an junior school”.

Abbott’s narrative was disputed by Andy Stone, a spokesman for Meta, who said on Twitter that the suspect’s messages were private one-to-one direct text messages discovered after the shooting occurred.

The company declined to say who received the messages or which of Meta’s platforms, such as Messenger or Instagram, was used to send them.

Abbott said 17 people suffered non-life-threatening injuries, including “multiple children” who survived the gunfire in their classroom, according to Chris Olivarez, a Texas Department of Public Safety spokesperson.

Investigators have not publicly identified a motive for what now ranks as the deadliest US school shooting in nearly a decade. Ten days earlier an avowed white supremacist shot 13 people at a supermarket in a mostly black neighbourhood of Buffalo, New York, has reignited a national debate over US gun laws.

In a sign of the charged political atmosphere, Beto O’Rourke, the Democratic candidate challenging Abbott in a November election, interrupted the news conference to confront Abbott over the state’s permissive gun laws, shouting “You are doing nothing!”

Several officials gathered on stage around the governor yelled at O’Rourke. “You’re a sick son of a b**** who would come to a deal like this to make a political issue,” one of them said, though it was not clear who.

Texas shooting among deadliest school attacks in past 10 years

O’Rourke was escorted out of the building and spoke to reporters outside. He said it was “insane” that an 18-year-old was legally permitted to acquire a semi-automatic rifle and vowed to pursue gun restrictions.

“We can get that done if we had a governor that cared more about the people of Texas than he does this own political career or his fealty to the NRA,” he said, referring to the National Rifle Association, a gun-rights advocacy organisation.

Abbott said stringent gun laws do not prevent violence, citing states such as New York. He said policymakers should instead focus on mental health treatment and prevention.

In a prime-time address on Tuesday evening, US President Joe Biden called for new gun safety restrictions.

“As a nation, we have to ask when in God’s name we’re going to stand up to the gun lobby,” he said, his voice rising.

But new legislation appeared unlikely to pass in Washington. Virtually all Republicans in Congress oppose gun restrictions, and there was no sign the massacre would alter that position.

White House officials were planning a trip to Texas for Biden, a senior administration official said.

The NRA’s annual meeting starts on Friday in Houston, where Republicans including Abbott, Texas US Senator Ted Cruz and former president Donald Trump are all scheduled to speak.

In a statement, the NRA expressed sympathy for the victims but said the event would go on as planned.

World leaders expressed shock and sorrow. Pope Francis on Wednesday said he was “heartbroken” and called for an end to “the indiscriminate trafficking of weapons”.

06:45

Horror and grief after Texas school shooting, Joe Biden calls for action

Horror and grief after Texas school shooting, Joe Biden calls for action

The head of the UN children’s agency Unicef, Catherine Russell, condemned the epidemic of US gun violence, comparing it to attacks on schools in Afghanistan, Ukraine and West Africa and blaming government leaders for inaction.

Shootings have become so commonplace in American schools that data shows a gun being fired almost every day this year on school property, according to the K-12 School Shooting Database at the Naval Postgraduate School’s Center for Homeland Defence and Security.

The Texas rampage stands as the deadliest US school shooting since a gunman killed 26 people, including 20 children, at Sandy Hook Elementary School in Connecticut in December 2012.

Uvalde, deep in the state’s Hill Country region, has about 16,000 residents, nearly 80 per cent of them Hispanic or Latino, according to US Census data.

As community members set up fundraisers for the families of the victims, some relatives mourned their loved ones on social media.

“My little love is now flying high with the angels above,” Angel Garza, whose daughter, Amerie Jo Garza, was killed, wrote on Facebook. “Please don’t take a second for granted. Hug your family. Tell them you love them.”

The two staff members killed were identified as Eva Mireles and Irma Garcia, fourth-grade teachers trapped in the classroom with their students when the shooting began.

Tribune News Service and Reuters