the school
https://www.npr.org/2022/07/18/1111940710/uvalde-texas-shooting-committee-report-arredondo-police
Relaxed school security allowed the gunman to attack quickly
Although Robb Elementary had safeguards and active shooter procedures in place, school staff had developed a culture of complacency around such measures. Out of convenience, some teachers frequently left doors unlocked or propped open — a violation of school policy. Due to a shortage of keys, substitute teachers were often told to circumvent locks.
The school was also set up with an intruder alert system. But the frequency of "bailout" alerts, which flag the presence of fleeing human traffickers in the area, desensitized teachers to their urgency. No prior bailout alert had ever resulted in a violent incident at the school.
On the day of the attack, the gunman scaled a 5-foot tall exterior fence before multiple unlocked doors allowed the gunman to enter the classrooms unimpeded, the report found.
"But had school personnel locked the doors as the school's policy required, that could have slowed his progress for a few precious minutes—long enough to receive alerts, hide children, and lock doors; and long enough to give police more opportunity to engage and stop the attacker," it read.
Instead, the gunman likely killed most of the victims before any responder entered the building, the committee found: "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."