Seven people are to be prosecuted in connection with the alleged ill-treatment of patients at Muckamore Abbey Hospital in Co Antrim.
The hospital is a care facility for adults with severe learning disabilities and mental health needs.
It is understood the seven individuals are staff members at the Cranfield Psychiatric Intensive Care Unit of the hospital.
The head of the PSNI's Public Protection Branch said the decision of the Public Prosecution Service (PPS) to prosecute them for a "range of offences" is "significant progress in the police investigation".
Detective Chief Superintendent Anthony McNally said this has been a "very detailed and complex investigation".
He said the PPS has written to the families involved to make them aware of the decision.
The seven people will be prosecuted for a range of offences including ill-treatment and wilful neglect contrary to the Mental Health (Northern Ireland) Order 1986.
"Our specially trained officers will continue to provide support to the families whose loved ones are at the centre of this investigation," Chief Supt McNally said.
"From the outset we have been committed to working closely with them, with the aim of keeping them as informed as much as we possibly can and we will continue to do so.
"The protection of our most vulnerable is a priority for the Police Service of Northern Ireland".
The PPS said the prosecutions relate to one police file from April 2020.
Its Assistant Director Martin Hardy said the decision follows a "thorough consideration of all the evidence" in this file.
A second police file from December 2020, also in relation to alleged ill-treatment at the hopsital is still being considered by the PPS.
It is understood that eight further suspects are under investigation in relation to this second file.
"Decisions will issue in due course after a thorough consideration of all the available evidence," Mr Hardy said.
"We acknowledge how difficult this process has been for the patients and their families, and we recognise the great dignity that they have shown. We will continue to engage with them and ensure they are kept informed as the process continues".
The PPS said that as proceedings against the seven people will "soon commence", it will not make any further comment.
The Belfast Health Trust, which operates the hospital, had no comment to make when contacted today.
