Detectives investigating hundreds of deaths at a hospital have identified 24 suspects.
An independent panel previously found 456 patients died after being given opiates inappropriately at Gosport War Memorial Hospital between 1987 and 2001.
Families of those who died have been informed a new criminal investigation, led by Kent Police, has begun sharing files with the Crown Prosecution Service for charging consideration.
Operation Magenta, which follows three previous investigations by Hampshire Constabulary that resulted in no prosecutions, said 21 people were being investigated for alleged gross negligence manslaughter and three for alleged health and safety offences.
Kent Police Deputy Chief Constable Neil Jerome said the investigation was “one of the largest and most complex of its nature in the history of UK policing”.
“It will be the CPS’ decision as to whether or not any criminal charges are brought in relation to these cases,” he added.
Police confirmed no-one had yet been arrested but interviews had been carried out under caution.
Representing some of the families affected, Emma Jones, from Leigh Day Solicitors, said the news was “small comfort” to her clients – who she praised for showing “immense patience and fortitude” while the investigation has been carried out.
“They have already waited many years for answers into the deaths of their loved ones and progress in this investigation does not appear to have been fast,” she said.
“We urge Kent Police to complete their investigations without further delay so that inquests into unexplained deaths at Gosport War Memorial Hospital can be re-opened as soon as possible.”
Officers working on Operation Magenta have so far looked at more than three million pages of documents, including the medical records of more than 750 patients.
About 1,200 witness statements from affected family members have also been taken, Kent Police said.
Families have previously campaigned for a judge-led “Hillsborough-style” inquest into the deaths.
Ms Jones repeated this call, saying it would mean they could “find the answers they deserve as efficiently as possible”.
A 2018 report into the deaths found there was a “disregard for human life” of a large number of patients from 1989 to 2000.
There was an “institutionalised regime” of prescribing and administering “dangerous” amounts of a medication not clinically justified at the Hampshire hospital, the report added.
Dr Jane Barton oversaw the practice of prescribing on the wards and is the only person to face disciplinary action.
She was found guilty of failings in her care of 12 patients between 1996 and 1999.
But she was not struck off the medical register, choosing to retire after the findings were published.
In a statement in 2018, Dr Barton said she was a “hard-working doctor” who was “doing her best” for patients in a “very inadequately resourced” part of the NHS.