One person who testified, said the door was unlocked (the police in fear thought it was locked) and the door could not be locked from the inside, though it could be locked with a key from the outside.
https://www.ksat.com/news/national/2022/06/21/texas-leader-says-uvalde-police-response-an-abject-failure/
https://www.ksat.com/news/local/2022/06/21/officer-husband-of-slain-uvalde-teacher-was-detained-had-gun-taken-away-after-trying-to-save-wife/
In the same testimony, Public Safety Director Col. Steven McCraw testified at the Texas State Senate that the teacher Eva Mireles who was shot and dying called her husband Ruben Ruiz who is a police officer for the school district. After the phone call the other police on the scene took the man's gun away, and spent time taking him off the scene while the gunman was still was live in the field.
Words can not capture my feelings of how dark this is