Four healthcare staff at Maghaberry Prison have been suspended after an investigation into the death of a prisoner in August.
John Anthony Deery, a convicted sex offender, was found in his cell and died later in the Royal Victoria Hospital in Belfast.
The Prisoner Ombudsman has been investigating the death.
The Prison Service Director General Robin Masefield said he was "deeply concerned" at what had been uncovered.
He said a disciplinary investigation had also been started.
"If the outcome of disciplinary investigation requires it, robust action will follow," he added.
"As the internal disciplinary investigation and the Ombudsman's investigation are both ongoing, it would be inappropriate to comment further at this stage."
A statement on behalf of the Ombudsman, Pauline McCabe, has confirmed she met with senior managers from the Prison Service and the South Eastern Health and Social Care Trust on Thursday to discuss areas of concern "identified in respect of the actions of a number of prison healthcare staff".
It said the investigation into Mr Deery's death was ongoing and a full report would be published in due course.
It added that Mrs McCabe met with the Deery family last week and they are aware of Thursday's meeting.
"The Prisoner Ombudsman has also confirmed that she is fully aware of positive action taken in recent weeks by the new Governor of Maghaberry Prison Steve Rodford, along with his management team, in respect of a range of challenges at Maghaberry, including the care of vulnerable prisoners," the statement said.
Deery, who was 50, died on 28 August.
He was jailed in 2003 after he admitted indecently assaulting five girls between 1975 and 2002.
He had served his sentence but shortly before his death he was recalled to prison for a breach of a probation order and was described as a "vulnerable" prisoner.
He was being held in a cell in the prison healthcare facility. It is thought he hanged himself.
A court hearing in 2003 was told Deery had attempted suicide while being held on remand and was at continuing risk of suicide while in custody.
Maghaberry prison has had a troubled and controversial recent history.
In July an inspection report said it was one of the worst prisons in the UK and also one of the most expensive to run.
The inspectors listed 200 ways to make the jail better and called for urgent action and better safety procedures.
The inspection took place in January, six months after an inmate hanged himself while under supervision in a specialist unit.
A damning report into that death lead to the governor and deputy governor of Maghaberry being moved from their jobs.
The current governor, Steve Rodford, had been in position for just a month when the latest death in custody occurred.
Last month dissident republican murder suspect Brian Shivers was released on bail after a court was told there had been "significant shortfalls" in his treatment for cystic fibrosis in Maghaberry prison hospital.