Detectives have identified 19 suspects in a probe investigating the deaths of more than 450 patients who had their lives cut short.
It comes nearly five years after an independent panel found hundreds of patients were given opiates without medical justification at Gosport War Memorial Hospital between 1987-2001.
A major police investigation involving 150 serving and retired officers has since assessed some three million pages of documents, including 750 patients' medical records.
Around 1,150 relatives of those who died have been interviewed for witness statements.
Police have also carried out several interviews under caution.
Deputy Assistant Commissioner Neil Jerome is leading the probe, named Operation Magenta, at the Kent and Essex Serious Crime Directorate.
Last year it emerged police were set to interview staff under caution.
He said: "The investigation is ongoing and continues to make good progress, with 19 suspects currently identified.
"The interviews under caution remain ongoing. Whilst we have never provided anyone with an estimate of how long our enquiries will last, family members can be confident we are working as quickly and thoroughly as possible to ensure Operation Magenta is the decisive police investigation into what happened at Gosport.
"Every single patient who died is important to us and their individual cases must be reviewed in full in order for an assessment to be made on their evidential strength.
"We also owe it to their families to investigate each death to the same high standard, and at the conclusion of the investigation all families who want to will be told everything we discovered about the nature of their care.
"It is these families who are at the heart of everything we do. We remain committed to building and maintaining trust and confidence among them and will continue to keep them updated on the progress of the investigation."
More than 450 people had their lives shortened at the hospital while another 200 were “probably” similarly given opioids between 1989 and 2000 without medical justification, according to the Gosport Independent Panel report released in 2018.
The independent report said there was “a disregard for human life and a culture of shortening lives of a large number of patients” at the hospital.
It said there was an “institutionalised regime of prescribing and administering ‘dangerous doses’ of a hazardous combination of medication not clinically indicated or justified”.
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