As the shotgun blows rained down, the floor inside the Luton block of council flats where Juliana Falcon lived on the with her teenage children was shaking.

A neighbour living on the floor below had already been upstairs to knock on the front door after hearing sounds of a struggle at around 5am on Friday 13 September last year.

Mariam Abbas could just about make out groaning and punching sounds inside the home as she rapped on the letterbox, but ran away in terror and called the police after hearing the distinctive bang of gunfire.

Little did she know that her 999 call, which led to the discovery of Ms Falcon’s slain body along with her two youngest children, would disrupt the plans of a teenage gunman who was determined to become one of the worst mass killer’s in history.

Nicholas Prosper, 18, had intended to shoot his family as they slept and lie in wait in their eighth floor flat before opening fire on 30 four and five-year-old children during assembly at a nearby primary school.

But a violent struggle with his family had alerted the neighbours and soon police were swarming the area.

Ms Falcon, 48, had been shot in the head along with her daughter Giselle Prosper, 13, and son Kyle Prosper, 16, who also suffered more than 100 stab wounds.

Prosper eventually gave up, flagged down a police car and was arrested. A loaded 12-bore Japanese Nikko shotgun he had bought with a forged licence was found hidden in bushes nearby, along with more than 30 cartridges intended for his meticulously planned attack.

If he had succeeded, it is feared he could have carried out the first major school shooting in Britain since the 1996 massacre in Dunblane, Scotland, which saw shooter Thomas Hamilton murder 16 children and a teacher before turning the gun on himself.

To horrified locals, there was nothing to suggest the quiet and reclusive teenager who spent most of his life in his bedroom, was plotting an atrocity.

“It shook the community up,” a neighbour told Sky News. “I think everybody has questions – why would this happen? How could a son do this to his own family?”

But analysis of his online world revealed a new side to the video-game obsessed teen, who appeared to be infatuated with an eight-year-old fictional character called Clementine from The Walking Dead.

In a video posted to his since-deleted social media account at 6.22am on the day of the attack, he claimed his sister Giselle had made “incorrect choices” while playing episode one of the zombie apocalypse game and “for that her face will be mutilated further than is necessary”.

He added: “I am the chosen one; chosen by Clementine. I am guided as Christians are guided by Jesus Christ.”

He is also said to have been banned from a webpage devoted to watching people die, described as “a community intended to observe and contemplate the very real reality of death” after making disturbing comments about child abuse, paedophilia and necrophilia.

Investigators found he had researched other mass school shooters, their manifestos and had an “interest in the darkest sides of humanity”, including people being killed or seriously injured.

Desperate for notoriety, he had also researched schools near his home and selected a distinctive black and yellow “uniform” to help him be remembered.

Last month, Prosper pleaded guilty to murdering his mother and two younger siblings in the bloody shooting, purchasing a shotgun without a certificate and possessing the gun and a knife.

On Wednesday, Prosper, now 19, was jailed for life with a minimum term of 49 years years at Luton Crown Court as a judge said his plans were “intelligent, calculated and selfish”.

In Bedford Prison, the killer – who has autism and “psychopathic traits” – has remained remorseless as he explained his actions to a medical practitioner, whispering to her: “I wish I had killed more.”

He told her he wanted to kill more than the 2012 Sandy Hook massacre in Connecticut, US, which claimed 28 lives, before taking his own life to “join” his friend Clementine from The Walking Dead.

The horrifying case comes amid calls for fresh legislation designed to target lone attackers in the wake of horrifying killings including the mass stabbing of girls at a Taylor Swift-themed dance class in Southport last year.

The prime minister said Britain faces a new threat from “extreme violence carried out by loners, misfits, young men in their bedrooms” as he called for the definition of terrorism to be expanded.

Home secretary Yvette Cooper said Prosper’s actions were the “most appalling crime that “raises a wider issue about young people becoming obsessed with violence”.

The UK’s terror watchdog, however, concluded in a report last week that it was right that such attacks are not classified as terrorism but called for new laws for those plotting mass casualty attacks which aren’t motivated by terrorism.

Jonathan Hall KC, the government’s Independent Reviewer of Terrorism Legislation, said in some cases it will be clear to investigators the attack was for “purely personal motives” but said “there is a real and not theoretical gap for lone individuals who plan mass killings.”

He warned it is “foreseeable” that other types of violent attacks will start a copycat craze, “most likely amongst the cohort of isolated often bullied teenagers with poor mental health, neurodivergence or personality disorder for whom grudges and grievances become reasons for violence”.

Referring to Prosper’s case, he added: “The key point to make is that terrorism legislation is not the UK’s main protection against this sort of attacker: what counts is gun control.”

In the aftermath of Prosper’s murders, tributes were paid to Ms Falcon, 48, who “lived for her children” and was described as “a positive person who will be so terribly missed”.

Shocked teachers at Lea Manor High School described Year 9 pupil Giselle “a beautiful soul and a model pupil” who will be sorely missed.

While a friend described Kyle, 16, who was passionate about football and boxing, as “really nice, confident, welcoming”.

Prosper’s father Ray, who was divorced from Argentinian-born Ms Falcon, said “part of his soul died too” when he learned of what happened in a brief victim personal statement. Prosper’s eldest brother, who lived with his father, was not caught up in the attack.

Despite the unspeakable tragedy, at an early court hearing Mr Prosper called out to his son: “I still love you, son. I know it’s not your fault, OK?”

After details of his school shooting plans emerged, St Joseph’s Catholic Primary School headteacher Maureen Murphy said all the children from the Prosper family had attended the school and “were a cherished part of our school community”.

“We were very shocked to learn that St Joseph’s was an intended target in this incident,” she added.

“However we would like to reassure our parents and carers that there is no threat to the school.”