UVALDE, TEXAS (NYTIMES) - Two minutes after a gunman burst through an unlocked door at Robb Elementary School and began shooting inside a pair of connected classrooms, Mr Pete Arredondo arrived outside, one of the first police officers to reach the scene.

The gunman could still be heard firing repeatedly, and Mr Arredondo, as leader of Uvalde's small school district police force, took charge. But there were problems from the start.

Mr Arredondo did not have a police radio with him, according to a law enforcement official familiar with the investigation, which may have impeded his immediate ability to communicate with police dispatchers.

As two supervisors from the local police department were grazed by bullets fired by the gunman, he made a decision to fall back, the official said.

Using a cellphone, Mr Arredondo called a police landline with a message that set the stage for what would prove to be a disastrous delay in interrupting the attack: The gunman has an AR-15, he told them, but he is contained; we need more firepower and we need the building surrounded.

Rather than confront an actively shooting gunman immediately, as officers have been trained to do since the killings at Columbine High School near Denver in 1999, the ever-growing force of increasingly armed officers arriving at Robb Elementary held back for more than an hour.

Old tactics

A New York Times examination of the police response - based on dozens of interviews with law enforcement officials, children who survived, parents who were witnesses outside, and experts on policing - found that breakdowns in communication and tactical decisions that were out of step with years of police preparations for school shootings may have contributed to additional deaths, and certainly delayed critical medical attention to the wounded.

A tactical team led by Border Patrol officers ultimately ignored orders not to breach the classroom, interviews revealed, after a 10-year-old girl inside the classroom warned 911 dispatchers that one of the two teachers in the room was in urgent need of medical attention.

The report that the incident commander at least initially had no police radio emerges as the latest important detail in what has been a shifting official account of the police response that has at times proved to be inaccurate on key points about the May 24 shooting.

Spokesmen for the Texas Rangers and the US Department of Justice, the two agencies now investigating the response, have said they would not be able to reach final conclusions until all interviews had been conducted and all available video and other evidence had been reviewed.

Officers who arrived at the scene, coming from at least 14 agencies, did not go into the classrooms as sporadic gunfire could be heard inside, nor after 911 calls began arriving from children inside. "There is a lot of bodies," a 10-year-old student, Khloie Torres, quietly told a 911 dispatcher at 12.10pm - 37 minutes after the gunman began shooting inside the classrooms - according to a review of a transcript of the call. "I don't want to die, my teacher is dead, my teacher is dead, please send help, send help for my teacher, she is shot but still alive."

She stayed on the line for about 17 minutes. Around 11 minutes into the call, the sound of gunfire could be heard.

'Do not breach'

The officers who finally breached the locked classrooms with a janitor's key were not a formal tactical unit, according to a person briefed on the response. The officers, including specially trained Border Patrol and Immigration and Customs Enforcement agents and a sheriff's deputy, had formed an ad hoc group on their own and gathered in the hallway outside the classroom, a tense space where they said there appeared to be no chain of command.

They were done waiting for permission, one of them said, according to the person, before they moved toward the classroom where the gunman waited. They continued even after one of them heard a command crackling in his earpiece: Do not breach.

They entered the room and killed the gunman.

The actions by Mr Arredondo and the array of officers he suddenly directed - which grew to number more than 140, from local, state and federal agencies, including state troopers, sheriff's deputies, constables and game wardens - are now the subject of overlapping investigations by the Texas Rangers, the Department of Justice and the local district attorney's office.

Mr Arredondo did not respond to multiple requests for comment.

Lockdown

In cases where a shooting drags on, and more experienced departments establish themselves at the scene, control may sometimes be handed over to a larger department. That did not happen in Uvalde, officials have said.

School-district police departments have jurisdiction over school campuses - in Uvalde, there are eight - as well as anywhere that school buses travel.

A review of the response in Uvalde shows that the school acted almost immediately after the gunman hopped a fence and approached Robb Elementary after crashing a pickup truck and firing shots outside.

Adam Pennington, an 8-year-old student, was in the front office when the school received what appeared to be the first alert.

"A phone call came in and said a man jumped the fence holding a gun," said Adam, who said he hurried to shelter under a table.

An employee on the campus used a cellphone to open a district security app, selecting a red "lockdown" button and a second button warning that there was an active shooter, according to Mr David Rogers, chief marketing officer for Raptor Technologies, the company that provides the security app.

That warning tool was part of an extensive effort to enhance security in the Uvalde school district, which also included two-way radios for "key staff", two new school district police officers and requirements that all classroom doors remain locked.

But Mr Arredondo had no police radio when he arrived, according to the latest information gathered in the investigation, and the door to the classroom where most of the killing occurred, Room 112, was unlocked when the gunman arrived.

The lockdown alert was sent at 26 seconds past 11.32am, about two minutes after the initial 911 call from outside the school. It triggered an immediate mass distribution of e-mails, text messages and notifications that included blaring alarms sent to the cellphones of other school employees, Mr Rogers said.

Less than a minute later, the gunman was already inside the school.

'Good night'

Khloie Torres had been watching a movie with her fourth-grade classmates in Room 112 when her teacher, Irma Garcia, told the class to go into lockdown.

Ms Garcia turned off the movie, and then rushed toward the classroom door to lock it. But she struggled to find the right key for the door. Gunfire could be heard in the hallways.

Ms Garcia finally got hold of the right key, but the gunman was already there.

"He grabbed the door, and he opened it," Khloie said.

Ms Garcia tried to protect her students. The gunman began firing.

Khloie hid under a table, listening to more gunshots.

"You'll die," the gunman said to the room. He shot one of Khloie's best friends, Amerie Jo Garza, and the other teacher in the class, Ms Eva Mireles.

Then the gunman said "Good night," Khloie said, and began firing at students across the classroom.

One child shouted, "I'm shot," catching the attention of the gunman. He came back to the spot where the child was lying and shot the student again, killing him, Khloie said.

Poor decision

Mr Arredondo arrived at 11.35am, as the first officers began moving into the hallway outside the classroom door.

Two minutes later, a lieutenant and a sergeant from the Uvalde Police Department approached the door and were grazed by bullets. Shortly after that, Mr Arredondo placed a phone call from the scene, reaching a police department landline.

He described the situation and requested a radio, a rifle and a contingent of heavily armed officers, according to the law enforcement official familiar with the initial response, who described it on condition of anonymity because the official was not authorised to publicly disclose the details.

The decision to establish a perimeter outside the classroom, a little over five minutes after the shooting began, shifted the police response from one in which every officer would try to confront the gunman as fast as possible to one where officers treated the gunman as barricaded and no longer killing.

Instead of storming the classroom, a decision was made to deploy a negotiator and to muster a more heavily armed and shielded tactical entry force.

"They made a poor decision, defining that as a hostage-barricade situation," said Mr Bill Francis, a former FBI agent who was a senior leader on the bureau's hostage rescue team for 17 years.

"The longer you delay in finding and eliminating that threat, the longer he has to continue to kill other victims."

911 calls

Inside, the gunman moved between the two adjoining classrooms.

After he left her room, Khloie said, she called out quietly, "Is anybody OK? Is anybody hurt?"

"Yeah," one classmate replied.

"Just be quiet, so he doesn't come back in here," Khloie remembered responding.

Another child asked for help getting Ms Garcia's body off her.

A boy in her class, Khloie said, was worried that the gunman would find them.

"He won't find us," she told him.

Shortly after noon, nearly half an hour after the first police officers had arrived, Khloie began dialling 911. She said she called over and over again.

By then, the first tactical teams had arrived, along with officers carrying long guns. Scores of other officers were outside the school, keeping frantic parents away and starting to remove children from other classrooms, pulling some through windows.

In video taken outside the school, Border Patrol agents could be seen donning specialised equipment at around 12.15pm.

Six minutes later, several shots were heard, the sound coming from inside the classroom.

Uvalde Mayor Don McLaughlin said in an interview with CNN that the gunman did not answer his telephone when a negotiator tried to call him.

'It was chaos'

In the hallway outside the classrooms, a throng of heavily armed law enforcement officers anxiously awaited instructions.

But frustrations were growing, particularly among members of a Border Patrol tactical unit, according to the person who was briefed on the team's response.

"No one entity or individual seemed to have control of the scene," the person said. "It was chaos."

After more than an hour, the ad hoc group of officers who had arrived ready to attack the gunman was growing impatient, and decided to move in.

One of the members - equipped with an earpiece and small microphone - quietly announced over the radio that the group was preparing to go into the classrooms. At that point, a voice responded, telling them not to breach the doors.

They ignored the directive.

As the agents entered, the gunman appeared to be ready for them, the person said. He fired.

They fired back, with at least one bullet striking him in the head. A bullet fragment also grazed the head of one of the Border Patrol agents.

As soon as the agents announced over the radio that the gunman had been killed, attention turned to treating the wounded.

The agents helped set up a triage system, as more officers and emergency medical workers descended on the classrooms, trying to stabilise the children who had been shot but were still alive.

At one point during the siege, one of the two children who called 911 had reported that at least eight or nine of the children in the two classrooms were still alive.

Khloie and her surviving classmates were rushed from the classroom. The bodies of 19 children were recovered, along with those of the two teachers. Seventeen people, including a third teacher, were wounded.

"I don't understand why somebody did not go in," said Khloie's mother, Jamie.

Children and teachers would have still been shot, she said, "but it would have been way less than 21."