This is the moment armed officers discovered the bodies of a mother and two of her children after all three had been murdered by her son.

Astonishing footage shows a Bedfordshire Police response team bursting into a flat in Luton after gunshots were heard on September 13 last year.

Police officers are seen deploying smoke grenades into the apartment, preparing for the possibility that the shooter might still be inside, while announcing that there are three victims.

The team find the bodies of Julianna Falcon, 48, Giselle Prosper, 13, and Kyle Prosper, 16, inside the property along with spent shotgun cartridges.

However, there was no trace of the fourth family member, nor the gunman, Nicholas Prosper, and no weapon was discovered, leading authorities to declare him as a critical incident on the loose.

This footage is part of the latest double episode of 24 Hours in Police Custody titled ‘Murder is No Game’, broadcasted in two installments on Channel 4 – one aired last night and another airing tonight at 9 pm.

It begins with armed response officers being shown entering the flat to search for a shooter after a 999 call reported that gunshots had been heard.

One officer warns: ‘Cover your ears, it’s going to be loud.’ The sounds of gunfire follow, which are actually from the smoke grenades being launched into the property.

The Bedfordshire Police armed response team enter the flat in Luton after gunshots were heard

Officers are seen firing smoke grenades into the flat in case the shooter was still inside

Outside the door, one officer puts up three fingers and says: ‘Three. He said third victim.’

An officer looks shocked when she hears there are three victims, saying: ‘Three? Oh my God’

The response team rushes inside as the smoke alarm blares, and an officer calls out: ‘Don’t move, heads down, stack on me. I’ve advanced right. There’s a deceased on the floor. Identified third victim. Proceed.’

Outside the door, an officer signals with three fingers and states: ‘Three. He mentioned a third victim.’ A colleague appears stunned and responds: ‘Three? Oh my God.’

Nicholas Prosper was jailed for life with a minimum term of 49 years in March at Luton Crown Court after he murdered Ms Falcon, Giselle and Kyle.

The 19-year-old triple murderer was also sentenced for weapons offences, having plotted a mass shooting at his former primary school.

The Solicitor General referred Prosper’s sentence to the Court of Appeal in April, but the bid to increase it to a whole-life order was dismissed in July.

The day before the killings, Prosper obtained a shotgun and 100 cartridges from a legitimate firearms dealer through a ‘meticulously forged’ gun licence, and planned to kill 34 people at a school, including 30 children.

He shot his mother in the early hours of September 13, placing a book named How To Kill Your Family on her legs, before shooting his sister.

Prosper then killed his brother, shooting him twice and stabbing him more than 100 times.

Prosper hid for more than two hours before flagging down police officers in a nearby street and showing them where he had hidden a loaded shotgun and 33 cartridges near playing fields.

Nicholas Prosper, 19, holding a plank of wood as a mock gun, pretending to shoot people

The block of flats in Luton, Bedfordshire, where armed police entered following the shooting

Juliana Prosper, 48, was found dead in the family home in Luton on September 13 last year

The Channel 4 documentary follows the development of the manhunt for Prosper and how detectives discovered his identity before probing his motivations.

When police finally arrested Prosper, he claimed he had not murdered his mother, brother and sister but also repeatedly asked: ‘Are the schools in lockdown?’

He also told police that he wished he had killed more people, and detectives realised that killing his family was only the first stage of a plan to become the most deadly mass school shooter in history.

Prosper would have become the first person aged under 21 to be given a whole-life order if his sentence had been increased. He is due to be released in his late 60s at the earliest.

Mrs Justice Cheema-Grubb sentenced him to life with a minimum term of 18 years for possession of a firearm with intent to endanger life, 3.5 years for buying the gun and one year for possession of the knife, to run concurrently.

Ms Falcon was said to have been a hard-working woman who cared for her children and had tried to get Prosper to get help when he was struggling at school.

Kyle Prosper had ‘his whole future ahead, he fought bravely for his life, but there was no contest with his older armed brother’, the judge told the court.

Giselle Prosper (left), 13, and Kyle Prosper (right), 16, were also found dead inside the property

Prosper was jailed for life with a minimum term of 49 years in March at Luton Crown Court

A court artist’s drawing of Nicholas Prosper in the dock at Luton Crown Court on March 18

Giselle, who was just 13, was described as ‘a smiling girl, no doubt the treasure of her parents and friends’.

Prosper later told a prison nurse he had wanted to cause ‘the biggest massacre in the 21st century’ by murdering his family and carrying out a mass shooting at his former primary school. He deliberately chose Friday 13 for the day of the attack.

Throughout secondary school up to the end of year 11 there had been no concerns about him – he was described as a quiet and geeky boy with a small group of friends who were into computers.

But once he began sixth form he stopped engaging with school staff or his family, and refused help from mental health workers.

His then-undiagnosed autism spectrum disorder meant he could not stay in mainstream education or hold down a job, and he became increasingly isolated, spending more time online and becoming obsessed with school shootings.

Defending, David Bentley KC said he had gone down ‘an internet wormhole’.

He plotted for months to kill his family and carry out the school shooting, even choosing a black and yellow uniform that he would wear for the killing spree.

24 Hours in Police Custody: ‘Murder is No Game’ episode two is on Channel 4 tonight at 9pm. Episode one aired last night but is available to stream online.