Uvalde, Texas - A damning Justice Department report on Thursday cited "critical failures" by Texas police who waited until 21 people had been killed by a mass shooter at a school before intervening.

Nineteen young children and two teachers were killed when a teenage gunman went on a rampage at Robb Elementary School in May 2022 in America's worst school shooting in a decade. Police eventually shot and killed the gunman.

Investigators "identified several critical failures," the report said, "the most significant failure was that the responding officers should have immediately recognized the incident as an active shooter situation."

"In summary, the response to the May 24, 2022, mass casualty incident at Robb Elementary School was a failure," said the critical incident review, which is more than 550 pages long.

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"The painful lessons detailed in this report are not meant to exacerbate an already tragic situation."

Police in Uvalde have been under intense scrutiny since it emerged that more than a dozen officers waited for over an hour outside classrooms where the shooting was taking place and did nothing as children lay dead or dying inside.

A total of 376 officers – border guards, state police, city police, local sheriff departments, and elite forces – responded to the massacre, a Texas state lawmakers' report said in July 2022.

But the situation was "chaotic" due to the officers' "lackadaisical approach" to subduing the gunman, the report charged.

In October 2022, the Uvalde Consolidated Independent School District said it was suspending the small police force that has responsibility for safety and security in the handful of public schools under its authority.

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School shootings have become a regular occurrence in a country where about a third of adults own a firearm, and regulations on purchasing even powerful military-style rifles are lax.

Polls show a majority of voters favor stricter controls on the use and purchase of firearms. However, the gun ownership lobby is highly influential, and the courts have ruled that the constitutional right to bear arms applies to private owners.

In June 2022, reform advocates secured a limited victory with the passage of legislation that demands enhanced background checks for younger buyers and provides federal cash for states introducing "red flag" laws that allow courts to temporarily remove weapons from those considered a threat.

Investigators compiling Thursday's report, which does not name the shooter, collected and reviewed "more than 14,100 pieces of data and documentation, including policies, training logs, body camera, and CCTV video footage."