The mass shooting at Robb elementary school in the small city of Uvalde, south Texas, began on Tuesday 24 May with a single shot.

An 18-year-old local man, who just a few weeks before had reached the age where he could legally buy a military-style assault rifle, sent ominous social media messages pre-dawn, including telling someone he was going to shoot his grandmother.

Around 11am, a neighbor of the grandmother heard a shot and saw Salvador Ramos run out of the front door of his grandparents’ home to a pickup truck.

He seemed panicked and struggled to get the Ford going, the neighbor, Gilbert Gallegos, said, then drove off in a spray of gravel.

Moments later, the grandmother, Celia Martinez Gonzales, 66, who is known as Sally, came out covered in blood after being shot in the face, crying out. She has survived and remained in the hospital on Monday.

By 11.28am the gunman had reached Robb elementary, half a mile away, and crashed the pickup in a ditch, authorities said.

At that moment, video shows a teacher entering the school through a door that the teacher had emerged from and propped open moments earlier.

That door was usually closed and locked, according to the school security protocol, but was ajar.

The gunman had an assault rifle and a backpack crammed with ammunition, and shot at two men who emerged from the funeral home right across the street from the school.

Some of the 21 people he ultimately killed, including 19 children and two teachers, are now in caskets in that same funeral home, waiting to be laid to rest in a town devastated by shock, grief – and also anger at police failings now being reviewed by the federal government and at inaction on gun safety laws by state and federal politicians.

He hopped over a fence and approached the school – still shooting – as panicked people nearby called the police.

Authorities initially said Ramos exchanged fire with a school police officer before entering the building, but they later said the officer was not actually on campus and “sped” back upon hearing of the incident.

But the officer initially headed for the wrong man, confronting someone who turned out to be a teacher, after passing Ramos, who crouched behind a vehicle.

The gunman slipped through the open door and into adjoining fourth-grade classrooms packed with nine, 10 and 11-year-olds, overseen by two teachers, at 11.33am, authorities said.

The Associated Press chronicled the incident based on law enforcement reports, records and interviews with Uvalde residents.

The shooter rapidly fired more than 100 rounds then moved into the adjacent room, where witnesses heard screams, more gunfire and morbid music being blared by Ramos.

Two minutes after he entered the school, three police officers followed and were quickly joined by four more.

Authorities said Ramos exchanged fire from the classroom with the officers in the hallway and two of them suffered “grazing wounds”.

After shots started ringing out, a cafeteria worker who had just finished serving chicken tacos to 75 third-graders said a woman shouted into the lunchroom: “Code black. This is not a drill!”

The employees didn’t know what “code black” meant but closed blinds, locked the doors and escorted children behind a stage, said the worker, who spoke on condition of anonymity to avoid publicity.

In the nearly half-hour after the first officers went inside, as many as 19 piled into the hallway, authorities said.

Meanwhile, children and teachers elsewhere in the building were trying to escape, some climbing out of windows.

Parent Javier Cazares had raced to Robb elementary, his daughter Jacklyn’s school, when he’d heard there was a shooting, leaving his truck running with the door open as he ran into the school yard. He is a gun owner but, in his rush, didn’t have it with him.

He saw about five officers helping people evacuate and about 20 minutes after arriving said he spotted officers coming with heavy shields for the first time.

In the chaos, he felt that time was simultaneously “going so fast and it was going so slow”.

But he added: “From what I saw, things could have been a lot different.”

One onlooker recalled a woman yelling at officers: “Go in there! Go in there!”

At 12.03pm, a girl called 911 and whispered that she was inside the classroom with the gunman.

Minutes later, the Uvalde school district posted on Facebook that all campuses were going into lockdown but that “the students and staff are safe in the buildings. The buildings are secure.”

The girl called 911 twice more, talking of multiple dead but also children still alive. There was sporadic gunfire, the authorities have said.

A further 34 minutes passed before Customs and Border Patrol (CBP) agents arrived, entered the classroom and killed Ramos, approximately 80 minutes after he’d entered the school.

The long wait in the hallway was “the wrong decision” Steven McCraw, head of the Texas department of public safety, said on Friday after days of conflicting information.

Cazares and relatives later spent three desperate hours at a hospital before being told Jacklyn, 10, had died.

A supporter of the right to bear arms afforded by the second amendment to the US constitution, Cazares said there should be stricter gun laws and selling the type of gun used in the carnage to an 18-year-old was “kind of ridiculous”.