Eight nursing homes owned by the same company were raided as police launched a probe into the death of 12 residents.
Officers swooped following tip-offs from social services and seized hundreds of pieces of evidence.
Adrian Tilley, whose mum Valerie, 78, died in one of the homes, was among relatives called to a summit by police.
He said: “We need to know what happened.”
The probe comes as health chiefs revealed they had launched a series of unannounced spot checks at a number of homes.
The deaths occurred at properties run by Sussex Health Care between April 2015 and June 2017.
The company has 20 homes, providing beds for 580.
Not all are for the elderly. Some care for adults with physical or learning difficulties and the death of a young adult is believed to be one of the cases being probed as police determine whether criminal activity took place.
On Friday, relatives of the 12 who died 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 – was at the meeting.
He said: “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 an opportunity to “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 County Hall in Horsham.
Relatives were told how 17 officers have been assigned to the case – which could stretch into 2019.
Documents seized at the homes will be examined to determine if gross negligence or wilful negligence was at play in relation to the deaths.
Some 43 staff members will also be questioned as witnesses, though this number could change as the investigation develops.
Deaths being probed could date back as far as April 13, 2015 – when a change in the law meant it became an offence for a care-worker to ill-treat or wilfully neglect an individual in their care.
While the case is ongoing, adult social services will not refer new patients to the homes in question, families were told.
Sussex Health Care is registered in Jersey under joint chairmen Shiraz Boghani and Doctor Shafik Sachedina.
Mr Boghani is also co-founder of the Splendid Hotel Group and was named Hotelier of the Year at the Asian Business Awards 2016.
Sussex Health Care achieved accreditation from the Health Quality Service (HQS) in 2002 and gained the Investors In People standard in 2003.
On its website, the company says it focuses on care for older people.
It says: “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, confirmed: “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”.
Sussex Police confirmed the investigation is still in the early stages and no arrests have been made.
A spokesperson said: “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: “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.”
Our Valerie died after two falls… what did they do to keep her safe?
Grandmother Valerie Tilley died after suffering two falls at one of the homes being probed, relatives have revealed.
Her sister Christine Harris and her son Adrian Tilley were left devastated by the death of the 78-year-old last November.
Valerie was admitted to Longfield Manor in July, having suffered with dementia for two years.
A treasured picture shows her sipping a cup of tea at the home in Billingshurst, West Sussex, last August. She lived there 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 again.
She was discovered unconscious by care staff, rushed to hospital with bleeding on the brain and died on November 7.
Christine said she and her nephew Adrian – one of Valerie’s two sons – need answers. She added: “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”.