Law enforcement officers in Uvalde, Texas, waited more than an hour to storm into the two classrooms where a gunman was holed up, even though on-scene supervisors knew some victims were trapped alive inside, The New York Times reports.
Citing its review of video footage and other material gathered by investigators, the Times said more than a dozen of the 33 children and three teachers who were originally in the two adjoining classrooms remained alive from the time gunfire began inside to when officers entered and killed the suspect an hour and 17 minutes later.
The school district police chief leading the response appeared from videos and other documentation to have agonised over how long it was taking to obtain protective gear to use when officers charged in, and to find a key to the classroom doors, the Times said.
"People are going to ask why we're taking so long," a man who investigators believe to be Pete Arredondo, chief of the Uvalde Consolidated Independent School District's police force, could be heard saying during the siege, according to a transcript of police body-camera footage.
"We're trying to preserve the rest of the life."
The May 24 attack at Robb Elementary School in Uvalde, a small town in Texas about 130 kilometres west of San Antonio, killed 19 students and two teachers, ranking as the deadliest US school shooting in almost a decade.
According to documents cited by the Times, one of the teachers died in an ambulance and three of the children died at nearby hospitals, heightening questions about whether more lives could have been saved had the victims been reached sooner.
The Texas Department of Public Safety (DPS) publicly acknowledged days later that as many as 19 officers had waited about an hour in a hallway outside classrooms 111 and 112 before a Border Patrol-led tactical team finally made entry.
DPS officials have said Arredondo made the choice to hold off on sending officers in to neutralise the gunman, believing the immediate threat to students inside had abated after an initial flurry of gunfire in the classrooms.
Two officers were grazed by bullets as they initially approached one of the classrooms, and no further attempt was made to confront the gunman for another 40 minutes, police have said.
The head of DPS, Steven McGraw, has said the delay was "the wrong decision", acknowledging at least two fourth-grade girls cowering inside the classrooms placed frantic, whispered phone calls to local emergency dispatchers pleading for police to send help.
It remains unclear whether Arredondo or other officers inside the school learned of those 911 calls, the Times said.
But the newspaper reported investigative materials show Arredondo and others at the scene became aware at some point that not everyone inside the classrooms was already dead.
One of six uniformed police officers on Arredondo's school district force, Ruben Ruiz, had rushed to the scene and informed supervisors that his wife, fourth-grade teacher Eva Mireles, was shot but still alive in one of the classrooms after she called him from inside, according to the Times. Mireles later died of her wounds.
The Times said Arredondo did not respond to several requests for comment on its article.