A 33-year-old man has been arrested in connection with abuse allegations at Muckamore Abbey Hospital in County Antrim.
The suspect was arrested by officers in Antrim on Monday.
He is the second person to be arrested after allegations were made about the physical and mental abuse of patients.
Last month, a 30-year-old man was arrested and later released on bail.
It is understood that more arrests are planned.
Muckamore Abbey provides treatment for people with severe learning disabilities and mental health needs.
It has been alleged a number of vulnerable men and women were abused there.
A total of 36 staff members, mainly nurses, have been suspended at the hospital since 2017 when allegations of ill treatment first began to surface.
They have been placed on precautionary suspension from the hospital while police investigations into abuse allegations continue.
All the CCTV footage from the Psychiatric Intensive Care Unit (PICU) has now been viewed, however, a PSNI spokesperson said that the viewing of footage from three further wards is about to start.
The allegations first arose in the PICU ward.
The PSNI has also confirmed that a sex-related offence was caught on CCTV footage.
Patients were in the ward at the time but were unharmed and there was no physical contact between any patients and the staff member in question.