Ministers and health officials are being kept informed of the inquiry which is being carried out by Nottinghamshire police officers.
Three members of staff have been dismissed and two more suspended at the Stonecross Lane Residential Unit in Mansfield.
The inquiry began after the Central Nottinghamshire Health Care NHS Trust received an anonymous letter alleging a woman in the unit had been mistreated by staff.
The unit which consists of two cottages is jointly administered with the Metropolitan Housing Association.
The supervising body, Nottinghamshire County Council's Social Services Department, was criticised by a judge recently for taking no action in the case of Leanne White, a three-year-old girl who was murdered by her step-father.
Mr David Brotherhood, the Trust's chief executive, said yesterday that two staff members, a trained nurse and a care assistant, had been suspended on full pay immediately.
Mr Brotherhood added that three other members of staff had been sacked, but only one for the type of incident under police investigation. The incident had involved the slapping of a patient.
A Nottinghamshire Social Services spokesman confirmed that the unit was registered with them and that they had responsibiity for ensuring the level of care was of an acceptable standard.
A police spokeswoman in Nottingham said the inquiry was 'expected to take a considerable time'.Reuse content