The Northern Ireland Prison Service has confirmed the death in custody of a 27-year-old prisoner.
The prisoner who had been in Magilligan Prison died on Monday morning (4 February) in hospital.
His next of kin have been informed.
A spokesman said: "As with standard procedure, the PSNI, Coroner and Prisoner Ombudsman have launched investigations into the death."
Ronnie Armour, Director General of the Northern Ireland Prison Service said: "On behalf of the Northern Ireland Prison Service, I would like to extend my sympathy to the family of the prisoner. My thoughts are with them at this difficult time."