Police made the "wrong decision" by failing to storm a primary school classroom in Uvalde, Texas, as a gunman killed 19 children inside, the top safety official has said.
"If I thought it would help, I would apologise," Steven McCraw said during a heated press conference on Friday.
Officers delayed entering because they did not believe it was still an "active shooter" situation, he said.
But pupils inside made multiple calls begging for police to come.
Mr McCraw confirmed there had been a 40-minute gap from the police unit's arrival at Robb Elementary School to the moment they decided to storm the classroom where the gunman had barricaded himself.
The senior officer on the scene decided to wait until the school janitor arrived with the keys because they thought that either "no kids were at risk" by then, or "no-one was living anymore".
The delayed response, combined with video footage showing frustrated parents outside the school being tackled and handcuffed by police while the gunman was still inside, has led to growing public anger and scrutiny of how law enforcement handled the situation.
Authorities have struggled to give a clear timeline of how events unfolded in Uvalde.
On Friday, Texas Governor Greg Abbott said he was "livid" that he had been "misled" about certain information, which he had made public in a news conference earlier this week.
"As everybody has learned, the information that I was given turned out in part to be inaccurate," he said.
He also revealed an anonymous donor had paid $175,000 (£139,000) for the funerals of the victims. "We appreciate that anonymous donor for his generosity," he said. "And we will ensure that those resources get into the right hands."
The gunman crashed his car near the school at about 11:30 local time, Mr McCraw disclosed, and walked around the building firing "more than one hundred rounds" into classrooms as he looked to get inside.
An officer for the school district, who was not on campus at the time, drove immediately to the scene following a 911 call but "drove right by the suspect who was hunkered down behind a vehicle", Mr McCraw said.
By 11:35, the assailant had entered the school through a door propped open earlier by a teacher, and barricaded himself into a classroom.
Police officers followed him into the building minutes later but remained in the hallway.
Mr McCraw confirmed that as many as 19 police officers had gathered outside the classroom but had made no immediate effort to get inside.
It was not until 12:51 that a tactical unit entered the classroom and killed him - about 75 minutes after the attack began.
The commanding officer on scene - the Uvalde school district's chief of police, who was not present at Friday's news conference - believed the situation was no longer one involving an "active shooter".
The description is at odds with the disclosure that at least four emergency 911 calls were made from within the school - some from children barricaded inside with the gunman - begging for police to come.
"With the benefit of hindsight where I'm sitting now, of course it was not the right decision. It was the wrong decision. There's no excuse for that," said Mr McCraw.
Growing emotional amid a barrage of angry questions following the admission, he called the mistakes "tragic".
The great-grandfather of 10-year-old Alexandria Rubio, who died in the attack, told the BBC he thought the police were "cowards".
Ruben Mata Montemayor said he had seen officers "walking" towards Robb Elementary during the attack. "If there was a danger at the school, why weren't they running?" he said.
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After the gunman was shot dead, police found as many as 1,657 rounds of ammunition and 60 magazines in his possession.
They later ascertained he had forewarned of some of his actions in private messages to a Facebook friend. It was earlier alleged he made the declarations - "I shot my grandmother" and "I'm going to shoot up a school" - as public posts on the platform.
Mr McCraw said the suspect had asked his sister to buy him a gun last September, but "she flatly refused".
In private chat messages with four people on Instagram earlier this year, he discussed buying a gun and asked questions about it.
One user responded: "Are you going to shoot up a school or something?"
"No, and stop asking dumb questions and you will see," came the reply.
US President Joe Biden and the first lady are due to travel to Uvalde on Sunday, his second trip to a community rocked by gun violence in less than two weeks.