While statistics show most of America’s “active shooter” incidents are over within minutes, it took police in Texas up to an hour to storm the classroom where gunman Salvador Ramos massacred 19 children and two teachers.

At a remarkable news briefing amid growing pressure on the authorities to explain the response time, a top official admitted it was the “wrong decision” for police at the scene in Uvalde to wait for armed backup before engaging.

While they waited, Ramos was locked inside a fourth-grade classroom and able to carry out America’s deadliest school shooting in almost a decade.

Questions over officers’ response grow – live updates from Texas shooting

This is the timeline of events on 24 May, according to the latest details from the Texas Department for Public Safety (TDPS), video footage from the scene, and 911 calls made by teachers and pupils.

11.27am

The classroom door at Robb Elementary School the suspect is known to have used is propped open by a teacher.

11.28am

Ramos crashes his grandmother’s pickup truck near the school at the junction of Nicolas Street and Geraldine Street.

He gets out of the vehicle and crosses the street to Hillcrest Memorial Funeral Home, where he shoots at two men.

The suspect misses both men, but one falls, before making it back to the funeral home.

Ramos then scales the fence to enter the school grounds.

11.30am

A teacher makes a 911 call to report a “crash and a man with a gun”.

11.31am

The suspect begins shooting through classroom windows as he walks along.

Meanwhile, a patrol vehicle reaches the funeral hall and a separate one arrives at the school.

An earlier police account claimed Ramos was “engaged” by a school resource officer inside the grounds, but South Texas’s regional director for the TDPS told reporters on Thursday that “he was not confronted by anybody” and there was no armed officer on the site at the time.

Steve McGraw, director of TDPS, said at a press conference on Friday that “sometimes witnesses get it wrong”, adding: “The bottom line is the officer was not on scene, not on campus, but drove immediately to the area.”

11.32am

Ramos fires multiple shots at the school.

11.33am

He then enters the school through the propped open door and begins shooting inside the classroom.

At least 100 rounds are audible from video evidence.

Previously the TDPS said Ramos entered the school at 11.40am.

11.35am

Three officers from the Uvalde Police Department (UPD) enter the school via the same door as the suspect.

Another four follow, including a deputy county sheriff.

Two receive “grazing wounds” before they find the door locked.

11.37am

Ramos fires another 16 rounds.

11.51am

Police sergeant and USB agents arrive.

A child at the school has told a US media outlet that their classmate was shot dead by Ramos after a police officer told them “yell if you need help” and his cry revealed his location.

Police evacuate children from elsewhere in the school and call for backup.

Distraught parents gather outside the school at the junction between Geraldine Street and Old Carizo Road – some try to enter the building and are restrained by police.

11.54am

Video footage from the scene shows parents criticising police, with one saying “you need to go in there”.

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1:08

Desperate relatives restrained by police

12.03pm

The first child rings 911 to say she is inside the classroom in a call that lasts one minute and 23 seconds.

12.07pm

A live stream filmed by a bystander shows several armed individuals wearing body armour outside the school entrance.

A crowd of people gather away from the school entrance, and police cars are seen on the site.

A helicopter is also seen flying above.

12.03pm

More officers arrive at the school, with up to 19 moving inside the hallway.

12.10pm

The first child to ring 911 rings again to say several people have died.

A US Marshal Service statement said they arrived on the scene at 12.10pm to “assist federal, state, and local law enforcement”.

According to the TDPS, BORTAC officers didn’t arrive for another five minutes.

12.13pm

First child caller rings 911 a third time.

12.15pm

Members of the Border Patrol Tactical Unit (BORTAC) arrive, some with shields.

12.16pm

First child caller rings 911 for a fourth time to say between eight and nine students are still alive.

12.19pm

A second child calls 911, but hangs up when instructed to by her classmate.

12.21pm

Ramos fires his gun again, seemingly towards the locked door. Some shots are heard on the second child’s 911 call.

Meanwhile, officers move down the hallway towards the door.

12.36pm

Second child caller rings back and is told to stay silent on the line, she whispers: “He shot the door.”

12.38pm

Live stream shows an ambulance outside the school entrance.

12.42pm

Live stream shows someone carrying a protective ballistic shield towards the school entrance.

Police are seen asking people to move behind the police tape, towards the funeral home.

Other officers can be seen outside the school entrance.

12.46pm

The live stream zooms in to show numerous people in uniform outside the school entrance.

The second child caller says she can hear police officers outside the classroom.

12.47pm

The second child caller says: “Please send the police now.”

12.49pm

Live stream shows a group of armed individuals dressed in green move towards the school entrance.

12.50pm

BORTAC officers unlock the door with keys provided by a janitor. Shots are heard on one of the 911 calls.

They move in and kill the suspect.

Speaking on the condition of anonymity, one law enforcement official has said Border Patrol agents struggled to open the classroom door and had to wait for the janitor to open the door with a key.

12.51pm

Emergency medical services audio picks up a message saying “shots fired”.

Live stream reveals a stretcher coming out of the school.

1.01pm

Two ambulances are seen outside the school entrance.

1.06pm

Uvalde Police Department confirm they have stopped Ramos.

Police’s first priority is to ‘move in and confront gunman’

According to the TDPS, first responders waited for backup from the Border Patrol tactical unit (BORTAC) before entering the classroom where Ramos was.

Previously in the US, waiting for specially trained tactical teams was the standard response to “active shooter” incidents.

But this changed after the 1999 Columbine High School shooting in Colorado, when 12 students and a teacher were killed while police waited an hour for a SWAT team to arrive.

Responders usually operate in teams of four, so the guidance changed to encourage the first four to pursue the suspect.

This has since been changed further to emphasize that police officers should do all in their power to stop the suspect shooting and taking any more lives.

Read more: Tributes as all 21 Texas victims named and pictured

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2:16

How the Uvalde school shooting unfolded

The Advanced Law Enforcement Rapid Response Training Center (ALERRT) at Texas State University is the largest active shooter response training provider in the US.

They are recognised by the FBI and are responsible for training police across Texas.

Their guidance states: “Law enforcement’s purpose is to stop the active shooter as soon as possible. Officers will proceed directly to the area in which the last shots were heard.”

It stresses that the “first priority is to move in… bypassing wounded and confronting the shooter”.

ALERRT also claims that “active shooter situations are often over within 10 to 15 minutes”.

An FBI study of 160 active shooter incidents in the US between 2000 and 2013 found that the majority ended within five minutes or less – and around half were over within two minutes or less.

According to current estimates provided by officials, the time between police arriving and the gunman being shot dead at Robb Elementary School was more than an hour.

Read more: Maps reveal scale of gun violence in America

‘They should have gone after him’

Contradictory accounts by different law enforcement agencies, including the TDPS, Uvalde Police and US Marshal Service, had caused confusion over the timeline.

And social media footage of parents pleading with officers to storm the classroom have led to accusations officials were too slow to confront the gunman.

Javier Cazares, whose fourth-grade daughter Jacklyn was killed in the shooting, said: “They say they rushed in… we didn’t see that.”

Read more: Girls survived by smearing blood on herself and pretending she was dead

Christopher Suprun, based in Dallas, is director of clinical operations at the 9/11 Foundation and was a paramedic responder on 9/11.

He told Sky News: “Part of the problem is we didn’t have a clear timeline. It’s critical with these sorts of incidents to have a single voice providing information to the community and the press.

“Not having that is unacceptable from a communications point of view, but also an incident management point of view.”

Michael Dorn, executive director of Safe Havens International, which works on school safety in the US, said timings can often remain unclear until “eight to 12 months” after the incident.

“The information we have a couple of weeks after an event is usually quite different than what we get in the first day or two. And even that is usually quite inaccurate.”

Read more: What it’s like to send your kids to school in America

On the apparent one-hour delay, Mr Suprun added: “There are reports the officers were afraid of the gun the shooter had.

“That’s an occupational hazard. That’s like a fireman saying they’re afraid of fire. If cops are afraid of a gun an 18-year-old can buy, we should be questioning if they should be on our streets.”

He also reaffirmed that official guidance does not condone waiting for backup.

“The fact is, those first two officers should have been going after the shooter – prosecuting him to a point where he couldn’t shoot at children any more because he was too focused on the police.”

Not doing that also meant there was far less chance of the victims surviving, he said.

“For generations we have talked about the golden hour of trauma care – the sooner you get there and get the patient to a trauma centre the better.

“They took too long, and it was absurd that they weren’t ready for this.”

Read more: Why attempts to change US gun law have been doomed to fail

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1:36

‘Wrong decision’ to wait outside Texas classroom

Uvalde Police chief Daniel Rodriguez has maintained officers responded “within minutes” of the incident.

On Thursday, Mr McGraw said: “The bottom line is law enforcement was there. They did engage immediately. They did contain (Ramos) in the classroom.”

But at the press conference on Friday, he admitted the “wrong decision” had been made not to breach the classroom door.

He said the on-scene commander had judged the situation as “no longer an active shooter situation” and that they were instead dealing with “a barricaded subject”.

“Obviously there were children in that classroom that were at risk, and it was, in fact, still an active shooter situation,” he told reporters.

“From the benefit of hindsight, of course, it was not the right decision, it was the wrong decision.”