Police raided eight nursing homes and seized hundreds of pieces of evidence after tip-offs from social services.
Officers are investigating the deaths of 12 people in care homes run by Sussex Health Care between April 2015 and June 2017.
Adrian Tilley was one of the bereaved relatives called to a police summit after his 78-year-old mother Valerie died.
Longfield Manor is being investigated along with other Sussex Health Care homes
He told the Mirror: ‘We need to know what happened.’
Health bosses have also launched a series of unannounced spot checks across care homes.
Sussex Health Care runs 20 homes and provides beds for 580, including younger patients with physical and learning difficulties.
The death of a young adult is believed to be one of the cases under investigation as police determine whether criminal activity took place.
On Friday relatives were called to a meeting by Detective Chief Inspector Till Sanderson.
Mr Tilley – whose mother died after two falls at the Longfield Manor care home in Billingshurst, West Sussex – said he became emotional when listening to tales from other grieving relatives.
Debbie Ivanova has confirmed that the Care Quality Commission is working with police to investigate deaths dating back to 2015
‘There were some terrible, tragic stories from other grieving family members in that meeting.
‘It was very emotional. We all entrusted these homes to look after our loved ones and we need to know if there was wrongdoing or not.’
Sussex Police said the meeting was called so officers could ‘meet family members and explain why police are involved’.
Representatives from West Sussex County Council, the NHS, Care Quality Commission and the Coroner for West Sussex also spoke to relatives at the meeting at County Hall in Horsham.
They were told how 17 officers have been assigned to the case – which could stretch into 2019.
Seized documents will be examined to determine if gross negligence or wilful negligence was a factor in the deaths.
In has been confirmed that 43 staff members will also be questioned as witnesses, though more may be called upon as the investigation develops.
The deaths under investigation could date back as far as April 13, 2015, when a legal change made it an offence for a care-worker to ill-treat or wilfully neglect a patient in their care.
As the case unfolds, adult social services will not refer new patients to the homes in question, relatives were told.
Sussex Health Care is registered in Jersey under joint chairmen Shiraz Boghani and Doctor Shafik Sachedina.
Mr Boghani also co-founded the Splendid Hotel Group and was crowned Hotelier of the Year at the Asian Business Awards 2016.
Families of care home residents who died were called to a summit to discuss the case’s progression
Sussex Health Care gained accreditation from the Health Quality Service (HQS) in 2002 and the Investors In People standard in 2003.
The company’s website says it focuses on care for older people.
‘All of our homes are staffed by highly trained and dedicated carers and most offer specialist nursing care 24 hours a day. We pay great attention to the overall quality of life at our homes.’
The Care Quality Commission says it is carrying out an investigation in conjunction with the police probe.
Debbie Ivanova, deputy chief inspector of adult social care at the Care Quality Commission, confirmed the body is working with police.
‘We carried out a series of unannounced inspections as a result of information of concern regarding SHC Clemsfold Group Ltd and SHC Rapkyns Group Limited that was passed to us by West Sussex County Council.’
She said the CQC would ‘publish full reports, detailing our findings and any enforcement action against the provider, as soon as we can’.
A spokesperson for Sussex Police said there have been no arrests at this early stage.
‘Detective Chief Inspector Till Sanderson invited families of people whose deaths we may be investigating to a private meeting at County Hall, Horsham.
‘It was an opportunity to meet family members and explain why the police are involved.
‘Partner agencies, together with the coroner, attended to answer any questions from the families which they are better placed to answer.’
A spokesperson for Sussex Health Care said the group’s main priority was the care of its patients.
‘Whilst the investigation is still under way it would not be appropriate for us to comment on its scope or on any specific actions that have been taken.
‘Our priority at all times is the wellbeing of residents and we are confident that our homes and services continue to provide high quality care and support on a daily basis.
‘We continue to work openly and transparently with the county, the clinical commissioning group and the police to assist with the investigation.’
Grandmother Valerie Tilley died after two falls at one of the homes under investigation.
Her sister Christine Harris and her son Adrian Tilley were devastated by the death of the 78-year-old in November.
After suffering with dementia for two years she was admitted to Longfield Manor in July.
A picture shows her sipping a cup of tea at the home in Billingshurst, West Sussex, last August, where she lived for four months before suffering a fall during the night on October 29.
Christine, 69, said: ‘She left her room and went wandering up two flights of stairs. It’s the sort of behaviour that is very common in people with dementia.
‘The carers found her trying to open the fire door and say she fell back and hurt herself.’
Valerie was taken to hospital but was discharged the following day.
She suffered a second fall three days later on November 1 after leaving her room in the night once again.
Staff found her unconscious and she was taken to hospital where she was found to be bleeding on the brain. She died on November 7.
Christine said she and her nephew Adrian – Valerie’s son – need answers.
‘We want to know what measures were put in place to protect Valerie after her first fall.’
Plumber Adrian, 56, said: ‘It was a relief to be invited to this meeting. I welcome the investigation into my mother’s death. Both Christine and I need to know what happened.’
A Care Quality Commission inspection of the home last October rated its service as ‘Good’ – but the issue of safety ‘required improvement’.