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 officers at the site of the attack.
Jury selection was underway in the trial against Gonzales, who faces 29 criminal counts of child endangerment - one for each of the 19 children who died and for the 10 students who survived.
US media reported that the indictment charges he “failed to engage, distract or delay the shooter” after hearing shots.
Arredondo faces 10 counts for allegedly delaying the official response.
Both men have pleaded not guilty.
Judge Sid Harle said he expected the trial in Corpus Christi, 320km from Uvalde, to last around two weeks, ABC News reported.
-Agence France-Presse