The gunman responsible for a deadly school shooting in Texas posted on social media minutes before the attack, the state governor has said.
Greg Abbott said gunman Salvador Ramos promised to shoot his grandmother and "shoot up a school".
Texas officials say Ramos was there for 40 minutes before he was killed.
Eyewitness Juan Carranza, 24, told the Associated Press that women shouted to officers to "go in there".
Javier Cazares, whose daughter was killed in the attack, told the agency he suggested running in with other onlookers because the police "aren't doing anything".
The shooting, which left 19 children and two adults dead, has reignited a long-running US debate on gun control.
Many have called for stricter gun buying laws, while others are sceptical that they would stop mass shootings.
No detail was given by investigators as to the motive of the attack.
According to CNN, the private messages were sent to a 15-year-old girl in Germany who Ramos had met online.
In a statement, Meta, Facebook's parent company, said the "private one-to-one text messages" were "discovered after the terrible tragedy occurred". It added that it was "closely co-operating" with investigators.
Tuesday's events in Uvalde - an unassuming town some 80 miles (129km) from San Antonio, America's seventh-largest city - brought the discussion once again to the fore, even as members of the small community sought to make sense of the tragedy.
Many there expressed divided attitudes about guns.
"As a kid, I remember my uncles teaching me and training me on how to hold a gun," Carlos Velasquez, a local resident, told the BBC.
"The juxtaposition of good safety with what just happened is so nuanced. It's not just a clean-cut thought - it's a really sticky situation and sticky conversation to have now," he said.
Others, however, were shocked that Ramos, 18, was able to carry out an attack with an AR-15 style semi-automatic rifle.
"This kid was just 18. You have to be 21 to drink. How?" asked Sandra Parra, who lives down the street from the school. "I hope there are changes," she said, referring to gun laws in Texas. "I don't have a gun myself, but if I did, it would be for protection," she added.
It is legal to buy a gun at 18 in Texas, and according to US media, the attacker bought his soon after his birthday.
Described as a loner from a "fraught home life", the gunman shot his grandmother before fleeing the scene in a battered truck carrying firearms and copious ammunition. He then drove erratically across town and crashed his car into a ditch near Robb Elementary School.
An officer engaged with him, but failed to stop him from entering the school. He then proceeded to shoot 19 children and two teachers dead, before officers converged on the classroom and a border patrol officer who had responded while nearby killed him, according to Mr Abbott.
A former classmate of the gunman, Ivan Arellano, 18, told the BBC that the shooter had always seemed "odd" and "anti-social" and bullied others, presumably to attract attention.
"A lot of people who knew him, we knew he wasn't mentally healthy," Mr Arellano said. "And a lot of people could agree that we probably should've said something."
However, Mr Abbott disclosed during the news conference that there had been no history to suggest the gunman could be a danger apart from the social media messages, sent less than an hour before the killings.
Speaking on Wednesday, US President Joe Biden said the idea that a teenager was able to legally purchase weapons that were "designed and marketed to kill, is just wrong".
"I'm just sick and tired of what's going on and what continues to go on," he said, calling for "action" on gun control.
His comments followed an incident at Mr Abbott's news conference, where the governor's Democratic challenger for office, Beto O'Rourke, heckled the Republican for not doing enough on gun control.
In response, Mr Abbott accused Mr O'Rourke of grandstanding.
According to the Texas Politics Project, only 43% of Texans support stricter gun laws. Nationally, the figure is 53% in support, according to a 2021 Pew Research Center poll.
As more details of the mass shooting emerged, the Uvalde community grieved.
The children killed were aged between seven and 10 years old. Teachers Eva Mireles and Irma Garcia also died in the attack. More than a dozen people were also wounded.
Vigils took place for the victims of the shooting, while people have been laying flowers near the school campus as tributes.
"The normality is not here anymore," Ms Parra said.
"Uvalde will be known for its mass shooting," said Mr Velazquez. "That's really unfortunate."