A Foyle MLA has called for a “full and transparent” investigation into the death of a Derry man in Maghaberry Prison this week.
Christopher Stokes (37) was found in a collapsed state in his cell on Monday night.
Staff tried to revive him but he was pronounced dead just before midnight.
Mr Stokes, who was originally from Derry, was given a minimum 20 year tariff last year for his part in the murder of John Mongan in west Belfast.
A father-of-three, Mr Mongan was killed in front of his heavily pregnant wife Julia in 2008. He suffered up to 50 wounds - some of those were possibly inflicted by a sword or machete.
The PSNI, Coroner and Prisoner Ombudsman were all notified and investigations were said to be ongoing.
Prison Service Acting Director General Max Murray said: “On behalf of the Northern Ireland Prison Service, I would like to extend my sympathy to the family. This is a tragedy and my thoughts are with them at this difficult time.”
Sinn Féin Assemblyman Raymond McCartney said prison chiefs needed to address “all the flaws in the system which can result in this type of tragedy immediately.”
“Without in any way apportioning blame, we have to reasssure ourselves that the failings within the prison system that have been consistently highlighted by several, knowledgeable agencies, including previous Prison Ombudsman’s Reports, have not been overlooked in respect to the circumstances around this case,” he said.
“Failings in our prison system have been highlighted by a number of reputable agencies and the last thing we need is another investigation resulting in recommendations that are not acted on with a sense of urgency.”