A report into the Uvalde school shooting in Texas that killed 21 people has found "systemic failures and egregiously poor decision-making" by those involved in the response.
The committee of state legislators highlighted a lack of leadership and urgency, describing a "lackadaisical approach" by authorities at the scene.
Nearly 400 officers rushed to the site, but police waited over an hour to confront the attacker.
The report was published on Sunday.
It was hand-delivered to victims' families before being made public.
On Monday, Texas state police announced an internal review into the 73 minutes of inaction by dozens of troopers who were at Robb Elementary School as an 18-year-old gunman slaughtered 19 children and two teachers on 24 May.
The Texas House of Representatives committee believes the nearly-80 page report to be the most complete telling so far of what happened during and after the attack.
It found no single "villain", other than the attacker, in the course of its investigation.
Instead, it concluded that there were multiple failures of responsibility from a number of authorities, including numerous law enforcement agencies and the school itself.
'Void of leadership'
The report heavily criticised the actions of the various agencies on the scene, accusing them of failing to prioritise "saving the lives of innocent victims over their own safety".
Despite nearly 400 officers rushing to the school, police waited well over an hour before confronting and killing the attacker - an "unacceptably long period of time" according to the report.
"We do not know at this time whether responders could have saved more lives by shortening that delay," the report adds.
The report also highlights a "void of leadership" - an apparent lack of anyone in charge which it says "could have contributed to the loss of life".
The Uvalde schools police chief Pete Arredondo wrote the district's active shooter response plan which assigns himself as incident commander, but testified that he did not consider himself to be in charge on the day.
The general view of witnesses interviewed for the report was either that Mr Arredondo was in charge, or that they could not tell who was in charge of a scene described by many as "chaos".
Mr Arredondo was placed on administrative leave last month and has since resigned.
However, the report points out that there were responders from numerous agencies on the scene - many better trained and better equipped than the school district police - who could have helped to take control of the situation.
Attacker entered 'unimpeded'
The criticism of the school centres around its lack of discipline in following security procedures designed to prevent such attacks.
The report said the school had a culture of leaving doors unlocked or propped open, sometimes to allow easier access for substitute teachers who didn't have keys.
The lock on room 111 - where much of the violence occurred - did not always work, the report found, but although the fault was widely known about it was not properly reported.
Because of these failures, the attacker was able to enter the school building and the classrooms unimpeded, and probably killed most of his victims before any responders set foot in the building.
"Of the approximately 142 rounds the attacker fired inside the building, it is almost certain that he rapidly fired over 100 of those rounds before any officer entered," the report says.
Earlier this week leaked CCTV footage from Uvalde was published by a local newspaper, showing the gunman's arrival and police waiting 77 minutes to confront him.
But it was published days before officials said they had planned to show the families or release it publicly.