POLICE were last night continuing to question a 30-year-old man in connection with abuse allegations at Muckamore Abbey Hospital in Co Antrim.
It is the first arrest by detectives probing the alleged physical and mental abuse of patients.
A PSNI spokesman said the man had been detained in the Antrim area yesterday morning.
The hospital, located outside Antrim town, is a regional facility for adults with severe learning disabilities.
Muckamore is currently at the centre of the biggest PSNI safeguarding investigation of its kind after CCTV footage emerged of alleged abuse of patients in a Psychiatric Intensive Care Unit (PICU) in September 2017.
A specialist police team has been trawling through thousands of hours of CCTV footage from the hospital.
To date 1,500 crimes have been identified on just one ward while 33 staff have been suspended by the Belfast health trust in relation to physical abuse and mental cruelty of patients with severe learning disabilities.
In an interview with the Irish News earlier this year, Detective Chief Inspector Jill Duffie described it as the "largest adult safeguarding case" of its kind undertaken by the PSNI.
Earlier this year, formal action was taken by the Regulation and Quality Improvement Authority (RQIA), when it issued three enforcement notices about staffing and nurse provision, adult safeguarding and patient finances.
The arrest comes after a leaked high-level report has revealed that allegations of abuse at Muckamore Abbey Hospital were first reported by whistleblowers seven years ago.
The Irish News yesterday revealed that the confidential document details a catalogue of appalling mistreatment of vulnerable patients with authors expressing concern that "alleged behaviours" were "happening openly in front of visiting staff" on a different ward, which is now closed, in November 2012.
Sources say the 2013 report, known as the 'Ennis Ward Adult Safeguarding Investigation', has re-surfaced within the top tiers of the trust over the past month, with questions being asked about its content and "how this could have happened again".
