A man has been arrested over allegations of ill-treatment of patients at Muckamore Abbey Hospital in Co Antrim.
The 33-year-old was detained by officers from the PSNI's Public Protection Branch in the Antrim area.
Officers have been investigating thousands of incidents of alleged abuse at the hospital for people with mental health needs and severe learning disabilities.
More than 30 staff members have been placed on precautionary suspension.
Detectives have been trawling through thousands of hours of CCTV from the hospital wards.
The footage has revealed an alleged 1,500 crimes on one ward, police have said.
A report was commissioned in September 2017, following reports of inappropriate behaviour and alleged physical abuse of patients by staff in two wards.
Reviewers raised concerns about the safeguarding of adults, provision of meaningful activities for patients and physical health care.
Northern Ireland Secretary Julian Smith previously apologised to families over the alleged abuse.
