The prisoner who died at HMP Pentonville after reportedly being stabbed with a hunting-style knife and thrown over fifth floor railings has been named as 21-year-old Jamal Mahmoud.
Mahmoud, who had just become a father, was convicted in July of this year of hiding a Skorpion machine gun in a woman's garden in Enfield.
He was said by the Met Police to be a member of the "Get Money Gang" based in Enfield, but was described by one person who knew him as "lovely" and "placid".
His death on Tuesday (18 October) at HMP Pentonville saw him plunge 30ft from the fifth floor of a prison wing, a prison source told the BBC.
He died at the scene while two other inmates injured during the incident remain in hospital.
Two prisoners, aged 34 and 26, have been arrested on suspicion of murder.
Police have not released any details of a possible motive for the attack or confirmed if it is gang-related.
Mahmoud's sister, Souzan, speaking on behalf of her mother Hawa, told ITV News: "For her to get a phone call that her son had died in prison is just devastating.
"I blame the prison more than the actual person that done it. They owed him a duty of care and they just neglected him. She's worried about my other brother, who is also in prison."
Both Mahmoud and another inmate convicted for the Skorpion firearms offence, 22-year-old Farah Mohamed, were said to be active members of the Get Money Gang – a notorious group which the Met Police says has operated for a number of years in the Enfield area.
Officers said the group had historically been involved in a number of violent incidents following turf war disputes.
The Skorpion machine-gun was found after the Met's Trident Gang Crime Command was executing a warrant at Mohamed's girlfriend's home in Dendridge Close, Enfield, in July 2015.
A small red case was discovered by officers containing a loaded Skorpian Machine Gun, with over 200 rounds of ammunition also recovered.
Mahmoud, who had also been handed a five-and-a-half year sentence for a separate robbery offence, admitted to having arranged for the Skorpion machine gun to be delivered. He was sentenced to six-and-a-half years, pleading guilty to three counts of conspiracy to transfer a prohibited weapon and one count of conspiracy to transfer prohibited ammunition.
His death has prompted calls for an inquiry to be held into the declining safety of prisons across England and Wales.