Seventeen patients at Stepping Hill Hospital in Stockport were affected by saline poisoning, police have revealed.
An investigation was launched by Greater Manchester Police in July after a nurse reported low blood sugar levels in patients at the hospital, part of Stockport Foundation Trust.
Three of the 17 patients thought to have been poisoned have died.
The police investigation has ruled out another 16 cases as being caused by poisoning. The investigation is ongoing, with a further nice cases still being looked at, a police spokesman said.
Assistant chief constable Terry Sweeney, who is in charge of the investigation told the BBC it would be hard to link the deaths with the contaminated insulin.
“It may well be that we don’t come to that outcome that we are looking for,” he said.
“But I can assure people that we will do everything we possibly can to make sure we do not miss the opportunities, because the victims who have been contaminated in this crime deserve that justice.”
He said the inquiry could take “months or even years” to conclude.
Registered nurse Rebecca Leighton was charged over the deaths and held for six weeks before being released in September when charges were dropped.
A Nursing and Midwifery Council ruling on 14 September allowed Ms Leighton to return to nursing. However, Ms Leighton admitted taking drugs from the hospital. Restrictions were imposed by the NMC which mean she cannot practise elsewhere and her access to drugs is restricted.