A 63-year-old woman was arrested in relation to allegations of patients’ ill-treatment at Muckamore Abbey Hospital.
Detectives from the PSNI’s Public Protection Branch arrested the woman in the Antrim area on Tuesday morning, making her the 27th person to be arrested with links to the allegations of abuse.
She was later released on Tuesday evening pending report to the Public Prosecution Service.
Reports of mistreatment at Muckamore Abbey Hospital - which is run by the Belfast Trust and located on the outskirts of Antrim - first came to light in 2017.
Police said they reviewed thousands of hours of CCTV footage as part of a major investigation, which has become the largest of its kind in Northern Ireland.
An alleged 1,500 crimes were discovered in one ward between April and September four years ago.
Seven people are also to face charges for a range of offences including ill-treatment and wilful neglect contrary to the Mental Health (Northern Ireland) Order 1986, in connection with the alleged ill-treatment of patients at the hospital.
A 56-year-old man was released last month after earlier being arrested in relation to the police probe, and a 53-year-old man was further released last week.
An inquiry into allegations of abuse at Muckamore Abbey Hospital officially began in October and is to be overseen by Tom Kark QC. Public hearings are set to be heard next spring.
The Co Antrim facility treats patients with severe learning difficulties and mental health problems.