Nineteen children and two members of staff were killed in the rampage at Robb Elementary on May 24, 2022 - one of the deadliest massacres at a school in American history.
Yet police officials who responded to the incident "demonstrated no urgency" in setting up a command post and failed to treat the killings as an active shooter situation, according to a damning report published on Thursday.
The US Justice Department report, the most comprehensive federal accounting of the haphazard police response, identifies a vast array of problems from failed communication and leadership to inadequate technology and training.
Federal officials say the issues contributed to the crisis lasting far longer than it should have - even as terrified students inside the classrooms called 911 and agonised parents begged officers to go in.
Attorney general Merrick Garland said in a statement the victims and survivors "deserved better".
"The law enforcement response at Robb Elementary on May 24, 2022 - and the response by officials in the hours and days after - was a failure," he said.
The mass shooting has already been the subject of intense scrutiny and in-depth examinations, but the nearly 600-page new report has provided better public understanding of how police in Uvalde failed to stop the attack.
The shooting has already been picked over in legislative hearings, news reports and a damning report by Texas legislators who faulted law enforcement at every level with failing "to prioritise saving innocent lives over their own safety".
Uvalde school district officers arrived within three minutes of the gunman's arrival at the school.
They ran towards the classroom he was in, but as they approached, the gunman fired from inside.
Two officers were hit by shrapnel and police retreated to take cover.
"An active shooter with access to victims should never be considered and treated as a barricaded subject," the report says, with the word "never" emphasised in italics.
A total of 376 officers were at the scene, including state police, Uvalde police, school officers and US Border Patrol agents.
A tactical team led by the Border Patrol eventually went into the classroom to take down the gunman, Salvador Ramos, roughly 77 minutes after police arrived on the scene.
In Texas, Republican governor Greg Abbott initially praised the officers' courage, and blame was later cast on local authorities in Uvalde.
But a report from a panel of state legislators and investigations by journalists laid bare how over the course of more than 70 minutes, a mass of officers went in and out of the school with weapons drawn but did not go inside the classroom where the shooting was taking place.
The federal report also details well-documented communication issues officials say hindered the response, including then-school district police chief Pete Arredondo discarding his radios on arrival because he thought they were unnecessary.
Although Mr Arredondo tried to communicate by phone with officers elsewhere in the school hallway, he told them not to enter the classrooms "because he appeared to determine that other victims should first be removed from nearby classrooms to prevent further injury".
Police also erred by failing to urgently establish a centralised command post, creating confusion among officers and even first responders, the federal report said.
The report includes a series of comments by terrified children taken from a 911 call, including: "Help!" "Help!" "Help!" "I don't want to die. My teacher is dead."
By that point, the students and their teachers had been trapped in classrooms with the gunman for 37 minutes, and the call lasted for 27 minutes.
Even though law enforcement officials were in the hallway and just outside the classrooms, it would be another 13 minutes after the call ended before the survivors were rescued.
In the 20 months since the Justice Department announced its review, footage showing police waiting in a hallway outside the fourth-grade classrooms where the gunman opened fire has become the target of national ridicule.
Mr Garland was in Uvalde on Wednesday ahead of the release of the report, visiting murals of the victims that have been painted around the centre of the town.
Later that night, Justice Department officials privately briefed family members at a community centre in Uvalde before the findings were made public.
The federal review was launched just days after the shooting, and local prosecutors are still evaluating a separate criminal investigation by the Texas Rangers.
Several of the officers involved have lost their jobs.
How police respond to mass shootings around the country has also been scrutinised since the tragedy.
The delayed response countered active-shooter training that emphasises confronting the gunman, a standard established more than two decades ago after the mass shooting at Columbine High School showed that waiting cost lives.
As what happened during the shooting has become clear, the families of some victims have condemned police as cowards and demanded resignations.
At least five officers have lost their jobs, including two Department of Public Safety officers and the on-site commander Mr Arredondo.