The videos emerging - along with initial accounts of the shooting timeline from authorities - suggest up to an hour passed between the time the attack began and the moment specially trained officers breached the fourth-grade classroom where 18-year-old Salvador Ramos had killed 19 children and two teachers, shooting him dead.
The New York Times reported that most, if not all, of the victims of the worst school shooting in nearly a decade likely died in the first few minutes of the attack, citing a person familiar with a preliminary timeline compiled by investigators.
A spokesperson for the Texas Department of Public Safety, Chris Olivarez, told CNN earlier on Thursday that investigators were still trying to confirm a precise minute-by-minute account, including how long Ramos remained barricaded inside the classroom.
In one video posted on Facebook by a man named Angel Ledezma, parents can be seen breaking through yellow police tape and yelling at officers to go into the building on Wednesday NZST.
"It's already been an hour, and they still can't get all the kids out," Ledezma said during the video.
Ledezma did not immediately respond to a request for comment.
Another video posted on YouTube showed officers restraining at least one adult. One woman can be heard saying, "Why let the children die? There's shooting in there."
"We got guys going in to get kids," one officer is heard telling the crowd. "They're working."
The shooting has reignited a national debate over the country's gun laws. President Joe Biden and his fellow Democrats have vowed to push for new restrictions, despite resistance from Republicans.
There were few warning signs: Ramos, a high school dropout, had no criminal record or history of mental illness. Governor Greg Abbott said on Wednesday that Ramos had written an online message to someone minutes before the attack saying he was about to "shoot up an elementary school."
Authorities said Ramos shot his grandmother in the face at the home they shared before fleeing and crashing his car outside Robb Elementary School sometime around 11.30 am. (1630 GMT). His grandmother, who is hospitalised in critical condition, managed to call police.
Outside the school, Ramos, armed with an AR-15 semi-automatic rifle, evaded a school police officer, dropping a bag full of ammunition as he did so. Officials have offered conflicting accounts as to whether Ramos and the officer exchanged gunfire before Ramos was able to enter the school.
"We're trying to establish exactly what was his role and how he encountered the shooter," Olivarez told CNN.
Ramos wounded two responding officers inside a hallway before barricading himself inside the classroom, Olivarez said. The attack drew a massive law enforcement response, including hundreds of officers, who encircled the building and broke windows in an effort to evacuate children and staff.
Eventually, members of a US Border Patrol tactical unit breached the classroom and apparently killed Ramos, with one agent wounded in the crossfire, officials said.
The Uvalde Police Department announced the incident was over shortly after 1 pm. (1800 GMT).
Olivarez said the attack lasted 40 to 60 minutes, though he couldn't say whether that time frame began when Ramos shot his grandmother or when the teenager arrived at the school. The FBI is working on obtaining surveillance video from school cameras.
At least 17 people were also injured, including children.
Victims' loved ones took to social media to express anguish over the loss of children who never came home from school.
"We told her we loved her and would pick her up after school," Kimberly Mata-Rubio posted on Facebook in a remembrance of her daughter, Alexandria Aniyah Rubio, a fourth-grade honor student. "We had no idea this was goodbye."