A videograb shows policemen in a hallway after Salvador Ramos entered Robb Elementary school in Uvalde, Texas. File/Reuters

A damning report and hours of body camera footage further laid bare the chaotic response to a mass shooting at a Uvalde elementary school, where hundreds of law enforcement officers massed but then waited to confront the gunman even after a child trapped with the shooter called 911.

The findings of an investigative committee released on Sunday were the first to criticise both state and federal law enforcement, and not just local authorities in the South Texas city for the bewildering inaction by heavily armed officers as a gunman fired inside two adjoining fourth-grade classrooms at Robb Elementary School, killing 19 students and two teachers.

Footage from city police officers' body cameras made public hours later only further emphasised the failures - and fuelled the anger and frustration of relatives of the victims.

"It’s disgusting. Disgusting,” said Michael Brown, whose 9-year-old son was in the school's cafeteria on the day of the shooting and survived. "They’re cowards.”

Nearly 400 law enforcement officials rushed to the school, but "egregiously poor decision making” resulted in more than an hour of chaos before the gunman was finally confronted and killed, according to the report written by an investigative committee from the Texas House of Representatives.

Together, the report and more than three hours of newly released body camera footage from the May 24 tragedy amounted to the fullest account to date of one of the worst school shootings in US history.

"At Robb Elementary, law enforcement responders failed to adhere to their active shooter training, and they failed to prioritise saving innocent lives over their own safety," the report said.

The gunman fired approximately 142 rounds inside the building - and it is "almost certain” that at least 100 shots came before any officer entered, according to the report, which laid out numerous failures. Among them:

- No one assumed command despite scores of officers being on the scene.

- The commander of a Border Patrol tactical team waited for a bulletproof shield and working master key for a door to the classrooms that may have not even been needed, before entering.

- A Uvalde Police Department officer said he heard about 911 calls that had come from inside the rooms, and that his understanding was the officers on one side of the building knew there were victims trapped inside. Still, no one tried to breach the classroom.

The committee didn’t "receive medical evidence” to show that police storming the classrooms sooner would have saved lives, but it concluded that "it is plausible that some victims could have survived if they had not had to wait 73 additional minutes for rescue.”

The findings had at least one immediate effect: Lt. Mariano Pargas, a Uvalde Police Department officer who was the city's acting police chief during the massacre, was placed on administrative leave.

Associated Press

Associated Press