A man has been arrested by detectives investigating allegations of ill-treatment of patients at Muckamore Abbey Hospital in Antrim.
The arrest comes amid a wide-ranging investigating into abuse allegations at the hospital, which cares for patients with severe learning disabilities and mental health needs.
An expert report was commissioned in September 2017 following reports of inappropriate behaviour and alleged physical abuse of patients by staff in two wards.
Northern Ireland Secretary Julian Smith previously apologised to families over the alleged abuse.
Police have said CCTV footage has revealed potential 1,500 crimes on one ward.
Detectives from the PSNI's Public Protection Branch arrested a 33-year-old man on Monday morning as part of the investigation.
Since 2017, almost 30 staff, mainly nurses, have been suspended at Muckamore.