A teenage gunman who slaughtered 19 kids and two teachers in a Texas school spent more than 40 minutes inside as witnesses desperately urged police to charge the building.
Salvador Ramos barricaded himself inside the classroom before opening fire on students and teachers inside.
Border Patrol agents finally breached the door about 40 minutes to an hour later when a staff member gave them a key.
Now a father of one of the dead children is blaming officers at the scene for not acting sooner to stop the gunman.
Grieving father Jacinto Cazares lost his 10-year-old daughter, Jackie Cazares when 18-year-old Ramos opened fire in the classroom.
It took police 90 minutes from when the first 911 call was placed, to the moment Ramos was shot and killed.
Cazares said had authorities followed the direction of witnesses and acted sooner, more lives could have been saved.
"There was at least 40 lawmen armed to the teeth but didn't do a darn thing [until] it was far too late," Jacinto Cazares, the father of 10-year-old victim Jackie Cazares, told ABC News.
"The situation could've been over quick if they had better tactical training, and we as a community witnessed it first-hand."
A witness who lived across the street from Robb Elementary, said onlookers begged officers outside the school to do something as bullets rang out inside the building.
"Go in there! Go in there!" nearby women shouted at the officers who did not go inside, 24-year-old Juan Carranza, said.
Local police, state police and 80 Border Patrol officers swarmed to the scene. Four of the 80 Border Patrol officers entered the school building and killed Ramos, according to a Customs and Border Protection official.
Carranza said he first watched Ramos crash his truck in a ditch outside the school, grab a AR-15-style semi-automatic rifle and shoot at, and miss, two people outside a nearby funeral home.
The mass shooter then fired at a school district security officer, ran inside the elementary school and shot at two arriving Uvalde police officers outside the building, according to a Texas Department of Public Safety spokesperson, who added that all three officers were injured.
Department of Public Safety Director Steve McCraw said it was "within 40 minutes or so" from when the 18-year-old opened fire on the school security officer to when the Border Patrol team shot him.
The specialised, SWAT-like team known as the Border Patrol Tactical Unit struggled to breach the classroom door and had to get a staff member to open it with a key, a law enforcement official who requested anonymity told the Associated Press.
Cazares said the officers needed better tactical training.
"I'm a gun owner and I do not blame the weapons used in this tragedy," he said.
"I'm angry how easy it is to get one and young you can be to purchase one."