Allegations of lack of care and safeguarding of 43 care home residents, 12 of whom have since died, are being investigated, police have confirmed.
Nine homes run by Sussex Health Care have been searched and medical records taken away, it has emerged.
The brother of one resident has told the BBC he raised concerns before and "wished people had listened".
The provider said it is "working openly with the police to support the investigation".
On Friday, relatives were called to a meeting with Sussex Police to discuss the investigation.
But Martyn Lewis, whose brother suffered a broken leg which the family feel was never properly explained, told BBC South Today they had raised concerns before.
He said: "Clearly the last two-and-a-half years have been an immense period of enquiry, challenge, pushback... And unfortunately the current events have basically proven - I think - that we were right.
"I wish people would have listened. It's about listening, it's about action. It's not about lessons learned."
Sussex Health Care, which runs 19 care homes for people with learning difficulties, neurological conditions and the elderly, told the BBC that incident was investigated and there was no evidence of wrongdoing.
Regarding the current inquiry, it said: "We are committed to assisting [the police] in any way we can and positively await the conclusion and the outcome of the investigation."
The Care Quality Commission carried out a number of unannounced inspections after it was alerted to the concerns by West Sussex County Council.
Due to "significant safeguarding concerns", it has identified two homes under investigation, both in Horsham, West Sussex - The Laurels in Guildford Road and Orchard Lodge in Dorking Road.
New placements have been suspended to eight homes run by the organisation.
Sussex Police said the investigation was at an early stage and no arrests had been made.
It added: "The investigation is... seeking to determine whether care standards fell to a criminal level of neglect, ill-treatment or negligence."
West Sussex County Council said it had no plans currently to move residents but would keep that under review.