Warning: This story contains content that may be distressing.
Outside of Robb Elementary School, a top Texas police official revealed how, days earlier, as many as 19 police officers waited 48 minutes to act as a gunman rampaged in a classroom.
From inside, desperate children and teachers repeatedly called 911 for help, he revealed.
"Obviously, based on the information we have, there were children in that classroom that were still at risk," said Steven McCraw, the director of the Texas Department of Public Safety.
"From the benefit of hindsight, where I'm sitting now, of course it was not the right decision. It was the wrong decision. Period."
The officers stood in the hallway for nearly an hour before using a master key to open a door to confront the armed teenager inside, Mr McCraw said.
By then, it was too late: 19 students and two teachers had been killed.
Police have offered conflicting accounts of how the shocking attack on a Uvalde elementary school unfolded around midday on Tuesday, local time.
What is clear is that the 18-year-old gunman intended to end lives, although his ultimate motive is unknown.
Here's what we know so far.
The gunman's final 90 minutes
At 11:28am, the gunman's pick-up truck slammed into a ditch behind the school building and the driver jumped out carrying an AR-15-style rifle.
According to authorities, 12 minutes later he entered the halls of Robb Elementary School and found his way to a fourth-grade classroom, where he opened fire.
But it wasn't until 12:58pm, some 90 minutes later, that law enforcement radio chatter said the gunman had been killed and the siege was over.
What happened in that period, in a working-class neighbourhood near the edge of town, has fuelled public anger over the police response.
"They say they rushed in," said Javier Cazares, whose daughter, Jacklyn Cazares, was killed while he and several other parents who rushed to the scene were left helpless.
"We didn't see that."
On Friday afternoon, Mr McCraw told reporters the on-site commander had believed the gunman was barricaded in a classroom and that the students were safe.
"He was convinced, at the time, that there was no more threat to the children and that the subject was barricaded and that they had time to organise" a response, he said.
"Of course it was not the right decision. It was the wrong decision."
Children called 911 for help
US Border Patrol agents eventually used a master key to open the locked door of the classroom where they confronted and killed the gunman, who had, by then, fatally shot 19 students and two teachers, according to Mr McCraw.
He said there was a barrage of gunfire shortly after officers entered the classroom where they killed the gunman, but it had been "sporadic" for much of the 48 minutes while they waited outside in the hallway.
Investigators have not yet established if or how many children died while they stalled.
Throughout the attack, teachers and children repeatedly called 911 asking for help, including a girl who pleaded: "Please send the police now," McCraw said.
One 11-year-old survivor of the attack played dead for almost an hour after smearing her body with a friend's blood, according to CNN.
The child told CNN, off-camera, that the gunman had backed a teacher into the classroom, began shooting, then entered the adjoining classroom.
Questions mount over slow police response
The updated attack timeline came after authorities were pressed on why officers had not stopped the gunman sooner.
On Thursday, Victor Escalon — the regional director for the Texas Department of Public Safety — told reporters he had "taken all those questions into consideration", but was not ready to answer them.
Rather than clarifying Tuesday's events, more troubling questions were raised about the time it took police to reach the scene and confront the gunman, and the apparent failure to lock a school door that the gunman had entered through.
After two days of providing often-conflicting information, investigators said a school district police officer was not inside the school when the gunman arrived, and, contrary to their previous reports, had not confronted him outside the building.
Instead, they sketched out a timeline filled with unexplained delays by law enforcement.
After crashing his truck, the gunman fired on two people coming out of a nearby funeral home, Mr Escalon said, before he entered the school "unobstructed" through an apparently unlocked door.
However, the first police officers did not arrive on the scene until 12 minutes after the crash and did not enter the school until four minutes after that.
Inside, they were driven back by gunfire from the gunman and took cover, Mr Escalon said.
The gunman was still inside at 12:10pm, when the first US Marshals Service deputies arrived.
They had raced to the school from nearly 113 kilometres away in the border town of Del Rio, according to an agency tweet.
The crisis came to an end when a group of Border Patrol tactical officers entered the school at 12:45pm, said a Texas Department of Public Safety spokesperson.
They engaged in a shootout with the gunman and moments before 1pm he was dead.
Mr Escalon said officers had called for backup, negotiators and tactical teams, while evacuating students and teachers.
Witnesses, families urged police to act
The motive for the massacre — the nation's deadliest school shooting since the Sandy Hook massacre in Connecticut, almost a decade ago — is under investigation, with authorities saying the Uvalde gunman had no known criminal nor recorded mental health history
During the siege, frustrated onlookers urged police officers to charge into the school, according to witnesses.
"Go in there! Go in there!" women shouted at the officers soon after the attack began, said Juan Carranza, who watched the scene from outside a house across the street.
Mr Carranza said the officers should have entered the school sooner.
"There were more of them. There was just one of him," he said.
Border Patrol Chief Raul Ortiz did not give a timeline but said, repeatedly, that the tactical officers from his agency who arrived at the school did not hesitate.
He said they moved rapidly to enter the building, lining up in a "stack" behind an agent holding up a shield.
"What we wanted to make sure is to act quickly, act swiftly, and that's exactly what those agents did," Mr Ortiz told Fox News.
Mr Cazares said that, when he arrived, he saw two officers outside the school and about five others escorting students out of the building.
However, 15 or 20 minutes passed before he saw officers arrive with shields, equipped to confront the gunman, he said.
As more parents flocked to the school, he and others pressed police to act but were ordered back to a parking lot.
"A lot of us were arguing with the police, 'You all need to go in there. You all need to do your jobs.' Their response was, 'We can't do our jobs because you guys are interfering,'" Mr Cazares said.
Community in mourning demands answers
Doug Swimmer, the pastor of The Potter's House, a local church, said the tragedy had turned the town upside-down, with several members of his congregation losing children.
"They want answers. They want to know the why," he told the ABC.
"And my fear is that, when the information comes out, it's not going to be what this community wanted to hear."
Following Mr McCraw's press conference, Texas's Governor Greg Abbott held his own, at first avoiding questions over law enforcement's changing timelines.
Mr Abbott said he had shared the information given to him by authorities earlier in the week and was "livid" that it had turned out to be "in part, inaccurate".
"I was misled," he said. "I am livid about what happened."
He said he would be demanding answers for "the families whose lives have been destroyed".
ABC/wires
