Police inquiries and unannounced inspections have been carried out at disability care homes in West Sussex.
They were in response to "significant safeguarding concerns" after a number of reported deaths.
The Care Quality Commission was alerted to Sussex Health Care services by West Sussex County Council.
The provider said it is "working openly with the police to support the investigation".
New placements have now been suspended to eight homes run by the organisation.
Two homes in Horsham were identified by the CQC as homes with "significant safeguarding concerns". The Laurels in Guildford Road - for young adults only - and Orchard Lodge in Dorking Road, cater for people with complex physical and learning difficulties.
'Investigation at early stage'
On Friday, Sussex Police met the families of people whose deaths they "may be investigating".
A spokesman said: "The police investigation still remains at an early stage and aims to identify whether any criminal offences have been committed, or not. No arrests have been made at this time.
"The investigation is examining the standard of care in individual cases and seeking to determine whether care standards fell to a criminal level of neglect, ill-treatment or negligence."
Sussex Health Care has been running for 30 years and has looked after about 30,000 people, it said.
A spokesman said: "Sussex Health Care is working openly with the police and West Sussex County Council to support their current investigation.
"We are committed to assisting them in any way we can and positively await the conclusion and the outcome of the investigation.
"Our priority at all times is to provide a high standard of care to every person we support.
"As a responsible provider, we have always invested in our staff and services to make sure we have the right resources available to provide that care."
West Sussex County Council spokesman said: "We have no plans currently to move residents but obviously we will continue to keep this under review.
"Robust safeguarding plans have been put in place for individual people and for the services they are using as is the case when safeguarding enquiries are raised.
Where other local authorities or organisations have placed people in the two named homes, the council is advising them of the concerns and asking them to review the placements.
Debbie Ivanova, deputy chief inspector of adult social care at the Care Quality Commission added that a full report, detailing CQC findings and any enforcement action against the provider, will be published once the investigation is over.