Two further staff members at Muckamore Abbey Hospital have been suspended, the health trust said.
Historical CCTV footage from the Co Antrim facility is being screened. The PSNI are also investigating.
Muckamore Abbey provides care for patients with learning disabilities or behavioural problems.
A report, which has not been made public by the Belfast Health and Social Care Trust, makes serious criticisms of the treatment of vulnerable adult patients.
A statement from the Trust said: “Belfast Trust will hold any individual staff member to account and take robust disciplinary action if their behaviour falls short of acceptable practice.
“We continue to view historical CCTV footage at Muckamore Abbey Hospital, and as of Friday two further members of staff at Muckamore Abbey Hospital have been suspended.
“As this process continues the Trust may have to take further action and we will comment when we are in a position to do so.”
The families of those affected met the permanent secretary at the Department of Health, Richard Pengelly, on Monday.
Mr Pengelly said: “It was important to me to apologise to families face-to-face for what happened to their loved ones while in the care of Muckamore Abbey Hospital – rather than through a press statement.
“I am both appalled and angered that vulnerable people were let down.
“At the same time, action is urgently needed by the HSC system as a whole in response to the recommendations of the Serious Adverse Incident (SAI) review.”
He said Muckamore would return to being a hospital providing acute care and not simply a residential facility.
“To make that happen will require investment in both specialised accommodation and staff training to meet the complex needs of people who no longer need to be in hospital.”
Mr Pengelly said he expects the resettlement process to be completed by the end of 2019.
That means finding suitable alternative accommodation for patients who have been living at Muckamore on a long-term basis, despite not requiring in-patient hospital care.
- Press Association