Corpus Christi, Texas - The trial of a former police officer charged with failing to stop the Uvalde school shooter who who killed 21 people in 2022, began on Monday.

The case against Adrian Gonzales is a rare example of an attempt to hold a law enforcement officer accountable for their actions during a mass shooting.

Two teachers and 19 young children were killed on May 24, 2022 when a teenage gunman went on a rampage with an AR-15 style assault rifle at Robb Elementary School, in what was America's deadliest school shooting in a decade.

Former school district police chief Pete Arredondo also faces charges over the tragedy, but will be tried separately.

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The official response was heavily criticized after 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.

The shooter, identified as 18-year-old Salvador Ramos, was reportedly killed by law enforcement at the site of the attack.

On Monday, a jury selection in Gonzales' trial was selected. He faces 29 felony counts of child endangerment – one for each of the 19 children who died and the 10 who survived.

The indictment reportedly alleges he "failed to engage, distract or delay the shooter" after hearing shots.

Arredondo faces 10 felony counts for allegedly delaying the official response.

Both men have pleaded not guilty.

Judge Sid Harle said he expected the trial in Corpus Christi, 200 miles from Uvalde, to last around two weeks, per ABC News.