At least two children placed several emergency calls from a pair of adjoining fourth-grade classrooms after 18-year-old Salvador Ramos entered on Tuesday with an AR-15 semi-automatic rifle, according to Colonel Steven McCraw, director of the Texas Department of Public Safety.
Ramos, who had driven to Robb Elementary School from his home after shooting and wounding his grandmother there, went on to kill 19 children and two teachers in the deadliest US school shooting in nearly a decade.
"He's in room 112," a girl whispered on the phone at 12:03 p.m., more than 45 minutes before a US Border Patrol-led tactical team finally stormed in and ended the siege.
The on-site commander, the chief of the school district's police department in Uvalde, Texas, believed at the time that Ramos was barricaded inside and that children were no longer at immediate risk, giving police time to prepare, McCraw said.
"From the benefit of hindsight where I'm sitting now, of course, it was not the right decision," McCraw said. "It was the wrong decision, period."
The disclosure of local law enforcement's delay in pursuing the teenaged gunman armed with a semi-automatic rifle came as the nation's leading gun-rights advocacy group, the National Rifle Association, opened its annual convention in Houston.
Governor Gregg Abbott, a Republican and staunch gun rights proponent who addressed the meeting in a pre-recorded video, seized on apparent police lapses in Uvalde, telling a news conference later he was misled and "livid about what happened."
Abbott denied newly enacted Texas gun laws, including a controversial measure removing licensing requirements for carrying a concealed weapon, had "any relevancy" to Tuesday's bloodshed.
He suggested state lawmakers focus renewed attention on addressing mental illness.
'Send the police now'
Even as the shooting reopened the intractable, long-running national debate over easy access to military-style weapons in the United States, the latest chronology of the Uvalde school attack stirred public dismay, including among the very officials reporting it.
McGraw, whose voice choked with emotion at times, said, "We're here to report the facts, not to defend what was done or the actions taken."
Some of the mostly 9- and 10-year-old students trapped with the gunman survived the massacre, including at least two who called 911, McCraw said. He did not offer a specific tally.
There were at least eight calls from the classrooms to 911 between 12:03 p.m., a half hour after Ramos first entered the building, and 12:50 p.m., when Border Patrol agents and police burst in and shot Ramos dead.
It was unclear whether officers at the scene were aware of those calls as they waited, McCraw said.
A girl whom McCraw did not identify called at 12:16 p.m. and told police that there were still "eight to nine" students alive, the colonel said. Three shots were heard during a call made at 12:21 p.m.
The girl who made the first call implored the operator to "please send the police now" at 12:43 p.m. and again four minutes later.
Officers went in three minutes after that final call, according to McCraw, when the tactical team used a janitor's key to open the locked classroom door.
Several officers had an initial exchange of gunfire with Ramos shortly after he entered the school at 11:33 a.m., when two officers were grazed by bullets and took cover. There were as many as 19 officers in the hallway by 12:03 p.m., when the first 911 call from inside the classroom was received, McCraw said.
Videos that emerged on Thursday showed anguished parents outside the school, urging police to storm the building during the attack, with some having to be restrained by police.
Standard law enforcement protocols call for police to confront an active school shooter without delay, rather than waiting for backup or more firepower, a point McCraw acknowledged on Friday.
Medical experts also stress the importance of evacuating critically wounded gunshot patients to a trauma centre within 60 minutes - what emergency physicians call "the golden hour" - in order to save lives.
McCraw described other moments when Ramos might have been thwarted. A school officer, responding to calls about an armed man who crashed a car at the funeral home across the street, drove right past Ramos as he crouched beside a vehicle on school property. Police have said Ramos fired at two people standing outside before scaling a fence onto school grounds.
The door that gave Ramos access to the building had been left propped open by a teacher, McCraw said, in violation of school district security policies.
Trump speaks to NRA convention
The attack, coming 10 days after a shooting in Buffalo, New York that left 10 people dead, has intensified the long-standing national debate over gun laws.
At the NRA meeting, prominent Republicans, including former President Donald Trump and US Senator Ted Cruz of Texas, doubled-down on arguments that tighter gun laws would do little or nothing to allay the rising frequency of US mass shootings.
About 500 protesters holding crosses, signs and photos of victims from the Uvalde shooting gathered outside the convention, shouting, "NRA go away."
President Joe Biden, a Democrat who has urged Congress to approve new gun restrictions, on Sunday will visit the community of 16,000 people about 130 km west of San Antonio.
Investigators are still seeking a motive for the attack. Ramos, a high school dropout, had no criminal record and no history of mental illness.